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| Boilerpipe Text | The Python programming language has a wide range of syntactical constructions, standard library functions, and interactive development environment features. Fortunately, you can ignore most of that; you just need to learn enough to write some handy little programs.
To accomplish this, however, youâll have to master some programming concepts. Like a wizard in training, you might think these concepts seem tedious, but with some practice, theyâll enable you to command your computer like a magic wand and perform incredible feats.
This chapter has a few examples that encourage you to enter code into the
interactive shell
, also called the
read-evaluate-print-loop (REPL)
, which lets you run (or
execute
) Python instructions one at a time and instantly shows you the results. Using the interactive shell is great for learning what basic Python instructions do, so give it a try as you follow along. Youâll remember the things you do much better than the things you only read.
Entering Expressions into the Interactive Shell
You can run the interactive shell by launching the Mu editor. This bookâs
introduction
provides setup instructions for downloading and installing it. On Windows, open the Start menu, enter
Mu
, and open the Mu app. On macOS, open your Applications folder and double-click
Mu
. Click the
New
button and save an empty file as
blank.py
. When you run this blank file by clicking the
Run
button or pressing F5, it will open the interactive shell, which will open as a new pane at the bottom of the Mu editorâs window. You should see a
>>>
prompt in the interactive shell.
You can also run the interactive shell from the command line Terminal (on macOS and Linux) or Windows Terminal (on Windows, where you could also use the older Command Prompt application). After opening these command line windows, enter
python
(on Windows) or
python3
(on macOS and Linux). Youâll see the same
>>>
prompt for the interactive shell. If you want to run a program, run
python
or
python3
followed by the name of the programâs
.py
file, such as
python blank.py
. Be sure you donât run
python
on macOSâs Terminal, as this may launch the older, backward-incompatible Python 2.7 version on certain versions of macOS. You may even see a message saying
WARNING: Python 2.7 is not recommended
. Exit the 2.7 interactive shell and run
python3
instead.
Enter
2
+
2
at the prompt to have Python do some simple math. The Mu window should now look like this:
>>>
2 + 2
4
>>>
In Python,
2
+
2
is called an
expression
, which is the most basic kind of programming instruction in the language. Expressions consist of
values
(such as
2
) and
operators
(such as
+
), and they can always
evaluate
(that is, reduce) down to a single value. That means you can use expressions anywhere in Python code that you could also use a value.
In the previous example,
2
+
2
is evaluated down to a single value,
4
. A single value with no operators is also considered an expression, though it evaluates only to itself, as shown here:
>>>
2
2
The Mu editor has a REPL button that shows an interactive shell with a prompt that looks like
In [1]:
. The popular Jupyter Notebook editor uses this kind of interactive shell. You can use this interactive shell the same way as the normal Python interactive shell with the
>>>
prompt. REPLs are not unique to Python; many programming languages also offer REPLs so that you can experiment with their code.
Programming involves some math operations you might not be familiar with:
Exponentiation (or
to the power of
) is multiplying a number by itself repeatedly, just like multiplication is adding a number to itself repeatedly. For example,
two to the power of four
(or
two to the fourth power
), written as or 2
4
or
2 ** 4
, is the number two multiplied by itself four times: 2
4
= 2 Ă 2 Ă 2 Ă 2 = 16.
Modular arithmetic is similar to the remainder result of division. For example,
14 % 4
evaluates to
2
because 14 divided by 4 is 3 with remainder 2. Even though Pythonâs modulo operator is
%
, modular arithmetic has nothing to do with percentages.
Integer division is the same as regular division except the result is rounded down. For example,
25 / 8
is
3.125
but
25 // 8
is
3
, and
29 / 10
is
2.9
but
29 // 10
is
2
.
You can use plenty of other operators in Python expressions too. For example, Table 1-1 lists all the math operators in Python.
Table 1-1:
Math Operators
Operator
Operation
Example
Evaluates to ...
**
Exponentiation
2 ** 3
8
%
Modulus/remainder
22 % 8
6
//
Integer division
22 // 8
2
/
Division
22 / 8
2.75
*
Multiplication
3 * 5
15
-
Subtraction
5 - 2
3
+
Addition
2
+
2
4
The
order of operations
(also called
precedence
) of Python math operators is similar to that of mathematics. The
**
operator is evaluated first; the
*
,
/
,
//
, and
%
operators are evaluated next, from left to right; and the
+
and
-
operators are evaluated last (also from left to right). You can use parentheses to override the usual precedence if you need to. Whitespace in between the operators and values doesnât matter in Python, except for the indentation at the beginning of the line. But the
convention
, or unofficial rule, is to have a single space in between operators and values. Enter the following expressions into the interactive shell:
>>>
2 + 3 * 6
20
>>>
(2 + 3) * 6
30
>>>
48565878 * 578453
28093077826734
>>>
2 ** 8
256
>>>
23 / 7
3.2857142857142856
>>>
23 // 7
3
>>>
23 % 7
2
>>>
2 + 2
4
>>>
(5 - 1) * ((7 + 1) / (3 - 1))
16.0
In each case, you as the programmer must enter the expression, but Python does the hard part of evaluating it. Python will keep evaluating parts of the expression until it becomes a single value:
These rules for gether to form expressions are a fundamental part of Python as a programming language, just like the grammar rules that help us communicate. Hereâs an example:
This is a grammatically correct English sentence.
This grammatically is sentence not English correct a.
The second line is difficult to parse because it doesnât follow the rules of English. Similarly, if you enter a bad Python instruction, Python wonât be able to understand it and will display a
SyntaxError
error message, as shown here:
>>>
5 +
File "<python-input-0>", line 1
5 +
^
SyntaxError: invalid syntax
>>>
42 + 5 + * 2
File "<python-input-0>", line 1
42 + 5 + * 2
^
SyntaxError: invalid syntax
You can always test whether an instruction works by entering it into the interactive shell. Donât worry about breaking the computer; the worst that could happen is that Python responds with an error message. Professional software developers get error messages all the time while writing code.
The Integer, Floating-Point, and String Data Types
Remember that expressions are just values combined with operators, and they always evaluate to a single value. A
data type
is a category for values, and every value belongs to exactly one data type. The most common data types
in Python are listed in Table 1-2. The values
-2
and
30
, for example, are said to be
integer
values. The integer (or
int
) data type indicates values that are whole numbers. Numbers with a decimal point, such as
3.14
, are called
floating-point numbers
(or
floats
). Note that even though the value
42
is an integer, the value
42.0
would be a floating-point number. Programmers often use
number
to refer to ints and floats collectively, although
number
itself is not a Python data type.
One subtle detail about Python is that any math performed using an int and a float results in a float, not an int. While
3
+
4
evaluates to the integer
7
, the expression
3
+
4.0
evaluates to the floating-point number
7.0
. Any division between two integers with the
/
division operator results in a float as well. For example,
16 / 4
evaluates to
4.0
and not
4
. Most of the time, this information doesnât matter for your program, but knowing it will explain why your numbers may suddenly gain a decimal point.
Table 1-2:
Common Data Types
Data type
Examples
Integer (int)
-2
,
-1
,
0
,
1
,
2
,
3
,
4
,
5
Floating-point number (float)
-1.25
,
-1.0
,
-0.5
,
0.0
,
0.5
,
1.0
,
1.25
String (str)
'a'
,
'aa'
,
'aaa'
,
'Hello!'
,
'11 cats'
,
'5'
Python programs can also have text values called
strings
, or
strs
(pronounced âstirsâ). Always surround your string in single-quote (
'
) characters (as in
'Hello'
or
'Goodbye cruel world!'
) so that Python knows where the string begins and ends. You can even have a string with no characters in it,
''
, called a
blank string
or an
empty string
. Strings are explained in greater detail in
Chapter 8
.
You may see the error message
SyntaxError: unterminated string literal
, as in this example:
>>>
'Hello, world!
SyntaxError: unterminated string literal (detected at line 1)
This error means you probably forgot the final single-quote character at the end of the string.
String Concatenation and Replication
The meaning of an operator may change based on the data types of the values next to it. For example,
+
is the addition operator when it operates on two integers or floating-point values. However, when
+
is used to combine two string values, it joins the strings as the
string concatenation
operator. Enter the following into the interactive shell:
>>>
'Alice' + 'Bob'
'AliceBob'
The expression evaluates down to a single, new string value that combines the text of the two strings. However, if you try to use the
+
operator on a string and an integer value, Python wonât know how to handle this and will display an error message:
>>>
'Alice' + 42
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' + 42
TypeError: can only concatenate str (not "int") to str
The error message
can only concatenate str (not "int") to str
means that Python thought you were trying to concatenate an integer to the string
'Alice'
. Your code will have to explicitly convert the integer to a string because Python cannot do this automatically. (Iâll explain how to convert between data types in
âDissecting the Programâ on page 14
, where we talk about the
str()
,
int()
, and
float()
functions.)
The
*
operator multiplies two integer or floating-point values. But when the
*
operator is used on one string value and one integer value, it becomes the
string replication
operator. Enter a string multiplied by a number into the interactive shell to see this in action:
>>>
'Alice' * 5
'AliceAliceAliceAliceAlice'
The expression evaluates down to a single string value that repeats the original string a number of times equal to the integer value. String replication is a useful trick, but itâs not used as often as string concatenation.
The
*
operator can only be used with two numeric values (for multiplication), or one string value and one integer value (for string replication). Otherwise, Python will just display an error message, such as the following:
>>>
'Alice' * 'Bob'
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' * 'Bob'
TypeError: can't multiply sequence by non-int of type 'str'
>>>
'Alice' * 5.0
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' * 5.0
TypeError: can't multiply sequence by non-int of type 'float'
It makes sense that Python wouldnât understand these expressions: you canât multiply two words, and itâs hard to replicate an arbitrary string a fractional number of times.
Expressions, data types, and operators may seem abstract to you right now, but as you learn more about these concepts, youâll be able to create increasingly sophisticated programs that do math on data pulled from spreadsheets, websites, the output of other programs, and other places.
Storing Values in Variables
A
variable
is like a box in the computerâs memory where you can store a single value. If you want to use the result of an evaluated expression later in your program, you can save it inside a variable.
Assignment Statements
Youâll store values in variables with an
assignment statement
. An assignment statement consists of a variable name, an equal sign (called the
assignment operator
), and the value to be stored. If you enter the assignment statement
spam
=
42
, a variable named
spam
will have the integer value
42
stored in it.
You can think of a variable as a labeled box that a value is placed in, but
Chapter 6
explains how a name tag attached to the value might be a better metaphor. Both are shown in Figure 1-1.
Figure 1-1: The code
spam = 42
is like telling the program, âThe variable
spam
now has the integer value 42 in it.â
For example, enter the following into the interactive shell:
âś
>>>
spam = 40
>>>
spam
40
>>>
eggs = 2
âˇ
>>>
spam + eggs
42
>>>
spam + eggs + spam
82
â¸
>>>
spam = spam + 2
>>>
spam
42
A variable is
initialized
(or created) the first time a value is stored in it
âś
. After that, you can use it in expressions with other variables and values
âˇ
. When a variable is assigned a new value
â¸
, the old value is forgotten, which is why
spam
evaluated to
42
instead of
40
at the end of the example. This is called
overwriting
the variable. Enter the following code into the interactive shell to try overwriting a string:
>>>
spam = 'Hello'
>>>
spam
'Hello'
>>>
spam = 'Goodbye'
>>>
spam
'Goodbye'
Just like the box in Figure 1-1, the
spam
variable in Figure 1-2 stores
'Hello'
until you replace the string with
'Goodbye'
.
Figure 1-2: When a new value is assigned to a variable, the old one is forgotten.
You can also think of overwriting a variable as reassigning the name tag to a new value.
Variable Names
A good variable name describes the data it contains. Imagine that you moved to a new house and labeled all of your moving boxes as
Stuff
. Youâd never find anything! Most of this bookâs examples (and Pythonâs documentation) use generic variable names like
spam
,
eggs
, and
bacon
, which come from the Monty Python âSpamâ sketch. But in your programs, descriptive names will help make your code more readable.
Though you can name your variables almost anything, Python does have some naming restrictions. Your variable name must obey the following four rules:
It canât have spaces.
It can use only letters, numbers, and the underscore (
_
) character.
It canât begin with a number.
It canât be a Python
keyword
, such as
if
,
for
,
return
, or other keywords youâll learn in this book.
Table 1-3 shows examples of legal variable names.
Table 1-3:
Valid and Invalid Variable Names
Valid variable names
Invalid variable names
current_balance
current-balance
(hyphens are not allowed)
currentBalance
current balance
(spaces are not allowed)
account4
4account
(canât begin with a number)
_42
42
(can begin with an underscore but not a number)
TOTAL_SUM
TOTAL_$UM
(special characters like
$
are not allowed)
hello
'hello'
(special characters like ' are not allowed)
Variable names are case-sensitive, meaning that
spam
,
SPAM
,
Spam
, and
sPaM
are four different variables. It is a Python convention to start your variables with a lowercase letter:
spam
instead of
Spam
.
Your First Program
While the interactive shell is good for running Python instructions one at a time, to write entire Python programs youâll enter the instructions into the file editor. The
file editor
is similar to text editors such as Notepad and TextMate, but it has some features specifically for entering source code. To open a new file in Mu, click the
New
button on the top row.
The tab that appears should contain a cursor awaiting your input, but itâs different from the interactive shell, which runs Python instructions as soon as you press
ENTER
. The file editor lets you enter many instructions, save the file, and run the program. Hereâs how you can tell the difference between the two:
The interactive shell will always be the one with the
>>>
or
In [1]:
prompt.
The file editor wonât have the
>>>
or
In [1]:
prompt.
Now itâs time to create your first program! When the file editor window opens, enter the following into it:
# This program says hello and asks for my name.
print('Hello, world!')
print('What is your name?') # Ask for their name.
my_name = input('>')
print('It is good to meet you, ' + my_name)
print('The length of your name is:')
print(len(my_name))
print('What is your age?') # Ask for their age.
my_age = input('>')
print('You will be ' + str(int(my_age) + 1) + ' in a year.')
Once youâve entered your source code, save it so that you wonât have to retype it each time you start Mu. Click
Save
, enter
hello.py
in the File Name field, and then click
Save
.
You should save your programs every once in a while as you type them. That way, if the computer crashes or you accidentally exit Mu, you wonât lose the code. As a shortcut, you can press
CTRL
-S on Windows and Linux or
ďş
-S on macOS to save your file.
Once youâve saved, letâs run our program. Press the F5 key or click the
Run
button. Enter your name when your program asks for it. The programâs output in the interactive shell should look something like this:
Hello, world!
What is your name?
>
Al
It is good to meet you, Al
The length of your name is:
2
What is your age?
>
4
You will be 5 in a year.
>>>
When there are no more lines of code to execute, the Python program
terminates
; that is, it stops running. (You can also say that the Python program
exits
.) The Mu editor displays the
>>>
interactive shell prompt after the program terminated, in case youâd like to enter some further Python code.
You can close the file editor by clicking the
X
on the fileâs tab, just like closing a browser tab. To reload a saved program, click
Load
from the menu. Do that now, and in the window that appears, choose
hello.py
and click the
Open
button. Your previously saved
hello.py
program should open in the file editor window.
You can view the step-by-step execution of a program using the Python Tutor visualization tool at
http://pythontutor.com
. Click the forward button to move through each step of the programâs execution. Youâll be able to see how the variablesâ values and the output change.
Dissecting the Program
With your new program open in the file editor, letâs take a quick tour of the Python instructions it uses by looking at what each line of code does.
Comments
The following line is called a
comment
:
# This program says hello and asks for my name.
Python ignores comments, and you can use them to write notes or remind yourself what the code is trying to do. Any text for the rest of the line following a hash mark (
#
) is part of a comment.
Sometimes programmers will put a
#
in front of a line of code to temporarily remove it while testing a program. This is called
commenting out
code, and it can be useful when youâre trying to figure out why a program isnât working. You can remove the
#
later when you are ready to put the line back in.
Python also ignores the blank line after the comment. You can add as many blank lines to your program as you want. This spacing can make your code easier to read, like paragraphs in a book.
The print() Function
The
print()
function displays the string value inside its parentheses on the screen:
print('Hello, world!')
print('What is your name?') # Ask for their name.
The line
print('Hello, world!')
means âPrint out the text in the string
'Hello, world!'
.â When Python executes this line, you say that Python is
calling
the
print()
function and the string value is being
passed
to the function. A value that is passed to a function call is an
argument.
Notice that the quotes are not printed to the screen. They just mark where the string begins and ends; they are not part of the string valueâs text.
NOTE
You can also use this function to display a blank line on the screen; call
print()
with nothing in between the parentheses.
When you write a function name, the opening and closing parentheses at the end identify it as the name of a function. This is why in this book, youâll see
print()
rather than
print
. Itâs a standard convention to have no spaces in between the function name and the opening parentheses, even though Python doesnât require this.
Chapter 3
describes functions in more detail.
The input() Function
The
input()
function waits for the user to type some text on the keyboard and press
ENTER
:
my_name = input('>')
This function call evaluates to a string identical to the userâs text, and the rest of the code assigns the
my_name
variable to this string value. The
'>'
string passed to the function causes the
>
prompt to appear, which serves as an indicator to the user that they are expected to enter something. Your programs donât have to pass a string to the
input()
function; if you call
input()
, the program will wait for the userâs text without displaying any prompt.
You can think of the
input()
function call as an expression that evaluates to whatever string the user typed. If the user entered
'Al'
, the assignment statement would effectively be
my_name
=
'Al'
.
If you call
input()
and see an error message, like
NameError: name 'Al' is not defined
, the problem is that youâre running the code with Python 2 instead of Python 3.
The Greeting Message
The following call to
print()
contains the expression
'It is good to meet you, '
+
my_name
between the parentheses:
print('It is good to meet you, ' + my_name)
Remember that expressions can always evaluate to a single value. If
'Al'
is the value stored in
my_name
, then this expression evaluates to
'It is good to meet you, Al'
. This single string value is then passed to
print()
, which prints it on the screen.
The len() Function
You can pass the
len()
function a string value (or a variable containing a string), and the function evaluates to the integer value of the number of characters in that string:
print('The length of your name is:')
print(len(my_name))
Enter the following into the interactive shell to try this:
>>>
len('hello')
5
>>>
len('My very energetic monster just scarfed nachos.')
46
>>>
len('')
0
Just like in those examples,
len(my_name)
evaluates to an integer. We say that the
len()
function call
returns
or
outputs
this integer value, and the value is the function callâs
return value
. It is then passed to
print()
to be displayed on the screen. The
print()
function allows you to pass it either integer values or string values, but notice the error that shows up when you enter the following into the interactive shell:
>>>
print('I am ' + 29 + ' years old.')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
print('I am ' + 29 + ' years old.')
TypeError: can only concatenate str (not "int") to str
The
print()
function isnât causing that error; rather, itâs the expression you tried to pass to
print()
. Youâll get the same error message if you type the expression into the interactive shell on its own:
>>>
'I am ' + 29 + ' years old.'
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'I am ' + 29 + ' years old.'
TypeError: can only concatenate str (not "int") to str
Python gives an error because the
+
operator can be used only to add two numbers together or to concatenate two strings. You canât add an integer to a string because this is not allowed in Python. You can fix this by using a string version of the integer instead, as explained in the next section.
The str(), int(), and float() Functions
If you want to concatenate an integer such as
29
with a string to pass to
print()
, youâll need to get the value
'29'
, which is the string form of
29
. The
str()
function can be passed an integer value and will return a string value version of the integer, as follows:
>>>
str(29)
'29'
>>>
print('I am ' + str(29) + ' years old.')
I am 29 years old.
Because
str(29)
evaluates to
'29'
, the expression
'I am '
+
str(29)
+
' years old.'
evaluates to
'I am '
+
'29'
+
' years old.'
, which in turn evaluates to
'I am 29 years old.'
This is the string value that is passed to the
print()
function.
The
str()
,
int()
, and
float()
functions will evaluate to the string, integer, and floating-point forms of the value you pass, respectively. Try converting some values in the interactive shell with these functions, and watch what happens:
>>>
str(0)
'0'
>>>
str(-3.14)
'-3.14'
>>>
int('42')
42
>>>
int('-99')
-99
>>>
int(1.25)
1
>>>
int(1.99)
1
>>>
float('3.14')
3.14
>>>
float(10)
10.0
The previous examples call the
str()
,
int()
, and
float()
functions and pass them values of the other data types to obtain a string, integer, or floating-point form of those values.
The
str()
function is handy when you have an integer or float that you want to concatenate to a string. The
int()
function is also helpful if you have a number as a string value that you want to use in some mathematics. For example, the
input()
function always returns a string, even if the user enters a number. Enter
spam
=
input('>')
into the interactive shell, then enter
101
when it waits for your text:
>>>
spam = input('>')
>
101
>>>
spam
'101'
The value stored inside
spam
isnât the integer
101
but the string
'101'
. If you want to do math using the value in
spam
, use the
int()
function to get its integer form and then store this as the variableâs new value. If
spam
is the string
'101'
, then the expression
int(spam)
will evaluate to the integer value
101
, and the assignment statement
spam
=
int(spam)
will be equivalent to
spam
=
101
:
>>>
spam = int(spam)
>>>
spam
101
Now you should be able to treat the
spam
variable as an integer instead of a string:
>>>
spam * 10 / 5
202.0
Note that if you pass a value to
int()
that it cannot evaluate as an integer, Python will display an error message:
>>>
int('99.99')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
int('99.99')
ValueError: invalid literal for int() with base 10: '99.99'
>>>
int('twelve')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
int('twelve')
ValueError: invalid literal for int() with base 10: 'twelve'
The
int()
function is also useful if you need to round a floating-point number down:
>>>
int(7.7)
7
>>>
int(7.7) + 1
8
You used the
int()
and
str()
functions in the last three lines of your program to get a value of the appropriate data type for the code:
print('What is your age?') # Ask for their age.
my_age = input('>')
print('You will be ' + str(int(my_age) + 1) + ' in a year.')
The
my_age
variable contains the value returned from
input()
. Because the
input()
function always returns a string (even if the user entered a number), you can use the
int(my_age)
code to return an integer value of the string in
my_age
. This integer value is then added to
1
in the expression
int(my_age)
+
1
.
The result of this addition is passed to the
str()
function:
str(int(my _age)
+
1)
. The string value returned is then concatenated with the strings
'You will be '
and
' in a year.'
to evaluate to one large string value. This large string is finally passed to
print()
to be displayed on the screen.
Letâs say the user enters the string
'4'
for
my_age
. The evaluation steps would look something like the following:
Description
The string
'4'
is converted to an integer, so you can add
1
to it. The result is
5
. The
str()
function converts the result back to a string, so you can concatenate it with the second string,
'in a year.'
, to create the final message.
The type() Function
Integer, floating-point, and string arenât the only data types in Python. As you continue to learn about programming, you may come across values of other data types. You can always pass these to the
type()
function to
determine what type they are. For example, enter the following into the interactive shell:
>>>
type(42)
<class 'int'>
>>>
type(42.0)
<class 'float'>
>>>
type('forty two')
<class 'str'>
>>>
name = 'Zophie'
>>>
type(name)
# The name variable has a value of the string type.
<class 'str'>
>>>
type(len(name))
# The len() function returns integer values.
<class 'int'>
Not only can you pass any value to
type()
, but (as with any function call) you can also pass it any variable or expression to determine the data type of the value that it evaluates to. The
type()
function itself returns values, but the angle brackets mean they are not syntactically valid Python code; you cannot run code like
spam
=
<class 'str'>
.
The round() and abs() Functions
Letâs learn about two more Python functions that, like the
len()
function, take an argument and return a value. The
round()
function accepts a float value and returns the nearest integer. Enter the following into the interactive shell:
>>>
round(3.14)
3
>>>
round(7.7)
8
>>>
round(-2.2)
-2
The
round()
function also accepts an optional second argument specifying how many decimal places it should round. Enter the following into the interactive shell:
>>>
round(3.14, 1)
3.1
>>>
round(7.7777, 3)
7.778
The behavior for rounding half numbers is a bit odd. The function call
round(3.5)
rounds up to
4
, while
round(2.5)
rounds down to
2
. For halfway numbers that end with .5, the number is rounded to the nearest even integer. This is called
bankerâs rounding
.
The
abs()
function returns the absolute value of the number argument. In mathematics, this is defined as the distance from
0
, but I find it easier to
think of it as the positive form of the number. Enter the following into the interactive shell:
>>>
abs(25)
25
>>>
abs(-25)
25
>>>
abs(-3.14)
3.14
>>>
abs(0)
0
Python comes with several different functions that youâll learn about in this book. This section demonstrates how you can experiment with them in the interactive shell to see how they behave with different inputs. This is a common technique for practicing the new code that you learn.
How Computers Store Data with Binary Numbers
Thatâs enough Python code for now. At this point, you might think that programming seems almost magical. How does the computer know to transform
2
+
2
into
4
? The answer is too complicated for this book, but I can explain part of whatâs going on behind the scenes by discussing what binary numbers (numbers that have only the digits 1 and 0) have to do with computing.
Hacking in movies often involves streams of 1s and 0s flowing across the screen. This looks mysterious and impressive, but what do these 1s and 0s actually mean? The answer is that binary is the simplest number system, and it can be implemented with inexpensive components for computer hardware. Binary, also called the
base-2 number system
, can represent all of the same numbers that our more familiar base-10
decimal number system
can. Decimal has 10 digits, 0 through 9. Table 1-4 shows the first 27 integers in decimal and binary.
Table 1-4:
Equivalent Decimal and Binary Numbers
Decimal
Binary
Decimal
Binary
Decimal
Binary
0
0
9
1001
18
10010
1
1
10
1010
19
10011
2
10
11
1011
20
10100
3
11
12
1100
21
10101
4
100
13
1101
22
10110
5
101
14
1110
23
10111
6
110
15
1111
24
11000
7
111
16
10000
25
11001
8
1000
17
10001
26
11010
Think of these number systems as a mechanical odometer, like in Figure 1-3. When you reach the last digit, they each reset to 0 while incrementing the value of the next digit over. In decimal, the last digit is 9, and in binary, the last digit is 1. Thatâs why the decimal number after 9 is 10 and the decimal number after 999 is 1000. Similarly, the binary number after 1 is 10 and the binary number after 111 is 1000. However,
10
in binary doesnât represent the same quantity as
ten
in decimal; rather, it represents
two
. And
1000
in binary doesnât mean
one thousand
in decimal, but rather
eight
. You can view an interactive binary and decimal odometer at
https://
inventwithpython
.com
/odometer
.
Figure 1-3: A mechanical odometer in decimal (left) and in binary (right)
Representing binary numbers with computer hardware is simpler than representing decimal numbers because there are only two states to represent. For example, Blu-ray discs and DVDs have smooth
lands
and indented
pits
etched on their surface that will or wonât reflect the disc playerâs laser, respectively. Circuits can have electric current flowing through them or no electric current. These various hardware standards all have ways of representing two different states. On the other hand, itâd be expensive to create high-quality electronic components that are sensitive enough to detect the difference between 10 different voltage levels with reliable accuracy. Itâs more economical to use simple components, and two binary states are as simple as you can get.
These binary digits are called
bits
for short. A single bit can represent two numbers, and 8 bits (or 1
byte
) can represent 2
8
, or 256, numbers, ranging from 0 to 255 in decimal or 0 to 11111111 in binary. This is similar to how a single decimal digit can represent 10 numbers (0 to 9), while an eight-digit decimal number can represent 10
8
or 100,000,000 numbers (0 to 99,999,999). Files on your computer are measured in how many bytes they take up:
A kilobyte (KB) is 2
10
or 1,024 bytes.
A megabyte (MB) is 2
20
or 1,048,576 bytes (or 1,024KB).
A gigabyte (GB) is 2
30
or 1,073,741,824 bytes (or 1,024MB).
A terabyte (TB) is 2
40
or 1,099,511,627,776 bytes (or 1,024GB).
The text of Shakespeareâs
Romeo and Juliet
is about 135KB. A high-resolution photo is about 2MB to 5MB. A movie can be anywhere from 1GB to 50GB, depending on picture quality and movie length. However, hard drive and flash memory manufacturers blatantly lie about what these terms mean. For example, by calling a TB 1,000,000,000,000 bytes instead of 1,099,511,627,776 bytes, they can advertise a 9.09TB hard drive as 10TB.
The 1s and 0s of binary can represent not only any integer but also any form of data. Instead of 0 to 255, a byte can represent the numbers â128 to 127 using a system called
twoâs complement
. Fractional floating-point numbers can be represented in binary using a system called
IEEE-754
.
Text can be stored on computers as binary numbers by assigning each letter, punctuation mark, or symbol a unique number. A system for representing text as numbers is called an
encoding
. The most popular encoding for text is UTF-8. In UTF-8, a capital letter
A
is represented by the decimal number 65 (or 01000001 as an 8-bit binary number), a
?
(question mark) is represented by the number 63, and the numeral character
7
is represented by the number 55. The string
'Hello'
is stored as the numbers 72, 101, 108, 108, and 111. When stored in a computer, âHelloâ appears as a stream of bits: 0100100001100101011011000110110001101111.
Wow! Just like in those hacker movies!
Engineers need to invent a way to encode each form of data as numbers. Photos and images can be broken up into a two-dimensional grid of colored squares called
pixels
. Each pixel can use three bytes to represent how much red, green, and blue color it contains. (
Chapter 21
covers image data in more detail.) But for a short example, the numbers 255, 0, and 255 could represent a pixel with the maximum amount of red and blue but zero green, resulting in a purple pixel.
Sound is made up of waves of compressed air that reach our ears, which our brains interpret as audio sensation. We can graph the intensity and frequency of these waves over time. The numbers on this graph can then be converted to binary numbers and stored on a computer, which later control speakers to reproduce the sound. This is a simplification of how computer audio works, but describes how numbers can represent Beethovenâs Symphony No. 5.
The data for several images combines with audio data to store videos. All forms of information can be encoded into binary numbers. There is, of course, a great deal more detail to it than this, but this is how 1s and 0s represent the wide variety of data in our information age.
Summary
You can compute expressions with a calculator or enter string concatenations with a word processor. You can even do string replication easily by copying and pasting text. But expressions, and their component valuesâoperators, variables, and function callsâare the basic building blocks that make up programs. Once you know how to handle these elements, you will be able to instruct Python to operate on large amounts of data for you.
Youâll find it helpful to remember the different types of operators (
+
,
-
,
*
,
/
,
//
,
%
, and
**
for math operations, and
+
and
*
for string operations) and the three data types (integers, floating-point numbers, and strings) introduced in this chapter.
I introduced a few different functions as well. The
print()
and
input()
functions handle simple text output (to the screen) and input (from the keyboard). The
len()
function takes a string and evaluates to an int of the number of characters in the string. The
str()
,
int()
, and
float()
functions will evaluate to the string, integer, or floating-point number form of the value they are passed. The
round()
function returns the rounded integer, and the
abs()
function returns the absolute value of the arguments.
In the next chapter, youâll learn how to tell Python to make intelligent decisions about what code to run, what code to skip, and what code to repeat based on the values it has. This is known as
flow control
, and it allows you to write programs that make intelligent decisions.
Practice Questions
â
â
1.â
â
Which of the following are operators, and which are values?
*
'hello'
-88.8
-
/
+
5
â
â
2.â
â
Which of the following is a variable, and which is a string?
spam
'spam'
â
â
3.â
â
Name three data types.
â
â
4.â
â
What is an expression made up of? What do all expressions do?
â
â
5.â
â
This chapter introduced assignment statements, like
spam
=
10
. What is the difference between an expression and a statement?
â
â
6.â
â
What does the variable
bacon
contain after the following code runs?
bacon = 20
bacon + 1
â
â
7.â
â
What should the following two expressions evaluate to?
'spam' + 'spamspam'
'spam' * 3
â
â
8.â
â
Why is
eggs
a valid variable name while
100
is invalid?
â
â
9.â
â
What three functions can be used to get the integer, floating-point number, or string version of a value?
10.â
â
Why does this expression cause an error? How can you fix it?
'I eat ' + 99 + ' burritos.'
Extra credit:
Search online for the Python documentation for the
len()
function. It will be on a web page titled âBuilt-in Functions.â Skim the list of other functions Python has, look up what the
bin()
and
hex()
functions do, and experiment with them in the interactive shell. |
| Markdown | (close)
Check out other books by Al Sweigart, [free online](https://inventwithpython.com/) or available for purchase:
[](https://nostarch.com/automate-boring-stuff-python-3rd-edition) [](https://nostarch.com/big-book-small-python-projects) [](https://nostarch.com/inventwithpython) [](https://nostarch.com/crackingcodes)
[...and other books as well\!](https://inventwithpython.com/) Or register for the [online video course.](https://inventwithpython.com/automateudemy) You can also [donate to support the author directly.](https://inventwithpython.com/#donate)
## 1 PYTHON BASICS
The Python programming language has a wide range of syntactical constructions, standard library functions, and interactive development environment features. Fortunately, you can ignore most of that; you just need to learn enough to write some handy little programs.
To accomplish this, however, youâll have to master some programming concepts. Like a wizard in training, you might think these concepts seem tedious, but with some practice, theyâll enable you to command your computer like a magic wand and perform incredible feats.
This chapter has a few examples that encourage you to enter code into the *[interactive shell](https://docs.python.org/3/tutorial/interpreter.html#interactive-mode)*, also called the *read-evaluate-print-loop (REPL)*, which lets you run (or *execute*) Python instructions one at a time and instantly shows you the results. Using the interactive shell is great for learning what basic Python instructions do, so give it a try as you follow along. Youâll remember the things you do much better than the things you only read.
### Entering Expressions into the Interactive Shell
You can run the interactive shell by launching the Mu editor. This bookâs introduction provides setup instructions for downloading and installing it. On Windows, open the Start menu, enter **Mu**, and open the Mu app. On macOS, open your Applications folder and double-click **Mu**. Click the **New** button and save an empty file as *blank.py*. When you run this blank file by clicking the **Run** button or pressing F5, it will open the interactive shell, which will open as a new pane at the bottom of the Mu editorâs window. You should see a \>\>\> prompt in the interactive shell.
You can also run the interactive shell from the command line Terminal (on macOS and Linux) or Windows Terminal (on Windows, where you could also use the older Command Prompt application). After opening these command line windows, enter python (on Windows) or python3 (on macOS and Linux). Youâll see the same \>\>\> prompt for the interactive shell. If you want to run a program, run python or python3 followed by the name of the programâs *.py* file, such as python blank.py. Be sure you donât run python on macOSâs Terminal, as this may launch the older, backward-incompatible Python 2.7 version on certain versions of macOS. You may even see a message saying WARNING: Python 2.7 is not recommended. Exit the 2.7 interactive shell and run python3 instead.
Enter 2 \+ 2 at the prompt to have Python do some simple math. The Mu window should now look like this:
```
>>> 2 + 2
4
>>>
```
In Python, 2 \+ 2 is called an *expression*, which is the most basic kind of programming instruction in the language. Expressions consist of *values* (such as 2) and *operators* (such as \+), and they can always *evaluate* (that is, reduce) down to a single value. That means you can use expressions anywhere in Python code that you could also use a value.
In the previous example, 2 \+ 2 is evaluated down to a single value, 4. A single value with no operators is also considered an expression, though it evaluates only to itself, as shown here:
```
>>> 2
2
```
The Mu editor has a REPL button that shows an interactive shell with a prompt that looks like In \[1\]:. The popular Jupyter Notebook editor uses this kind of interactive shell. You can use this interactive shell the same way as the normal Python interactive shell with the \>\>\> prompt. REPLs are not unique to Python; many programming languages also offer REPLs so that you can experiment with their code.
ERRORS ARE OKAY\!
The best thing about computers is that they carry out the exact instructions you give them. This is also the worst thing about computers. Computers canât use common sense to figure out what you intended to do. Programs will crash if they contain code the computer canât understand, which will cause Python to show an error message. Error messages donât damage your computer, though, so donât be afraid to make mistakes. A crash just means the program unexpectedly stopped running.
Get used to seeing error messages, because youâll constantly encounter them (even if you have decades of programming experience). Error messages are often vague and not meant to be immediately understood by beginners. If you want to know more about an error, you can search for the exact error message text online for more information. Note that if you are using the Mu editor, the keyboard shortcut for copying highlighted text in the interactive shell pane to the clipboard is CTRL-SHIFT-C, while the shortcut for copying text in the main file editor pane is the standard CTRL-C. You can also check out the resources at [https://nostarch.com/automate-boring-stuff-python-3rd-edition](https://nostarch.com/automate-boring-stuff-python-3rd-edition) to see a list of common Python error messages and their meanings.
Programming involves some math operations you might not be familiar with:
- Exponentiation (or *to the power of*) is multiplying a number by itself repeatedly, just like multiplication is adding a number to itself repeatedly. For example, *two to the power of four* (or *two to the fourth power*), written as or 24 or 2 \*\* 4, is the number two multiplied by itself four times: 24 = 2 Ă 2 Ă 2 Ă 2 = 16.
- Modular arithmetic is similar to the remainder result of division. For example, 14 % 4 evaluates to 2 because 14 divided by 4 is 3 with remainder 2. Even though Pythonâs modulo operator is %, modular arithmetic has nothing to do with percentages.
- Integer division is the same as regular division except the result is rounded down. For example, 25 / 8 is 3\.125 but 25 // 8 is 3, and 29 / 10 is 2\.9 but 29 // 10 is 2.
You can use plenty of other operators in Python expressions too. For example, Table 1-1 lists all the math operators in Python.
| Operator | Operation | Example | Evaluates to ... |
|---|---|---|---|
| \*\* | Exponentiation | 2 \*\* 3 | 8 |
| % | Modulus/remainder | 22 % 8 | 6 |
| // | Integer division | 22 // 8 | 2 |
| / | Division | 22 / 8 | 2\.75 |
| \* | Multiplication | 3 \* 5 | 15 |
| \- | Subtraction | 5 - 2 | 3 |
| \+ | Addition | 2 \+ 2 | 4 |
The *order of operations* (also called *precedence*) of Python math operators is similar to that of mathematics. The \*\* operator is evaluated first; the \*, /, //, and % operators are evaluated next, from left to right; and the \+ and \- operators are evaluated last (also from left to right). You can use parentheses to override the usual precedence if you need to. Whitespace in between the operators and values doesnât matter in Python, except for the indentation at the beginning of the line. But the *convention*, or unofficial rule, is to have a single space in between operators and values. Enter the following expressions into the interactive shell:
```
>>> 2 + 3 * 6
20
>>> (2 + 3) * 6
30
>>> 48565878 * 578453
28093077826734
>>> 2 ** 8
256
>>> 23 / 7
3.2857142857142856
>>> 23 // 7
3
>>> 23 % 7
2
>>> 2 + 2
4
>>> (5 - 1) * ((7 + 1) / (3 - 1))
16.0
```
In each case, you as the programmer must enter the expression, but Python does the hard part of evaluating it. Python will keep evaluating parts of the expression until it becomes a single value:
These rules for gether to form expressions are a fundamental part of Python as a programming language, just like the grammar rules that help us communicate. Hereâs an example:
This is a grammatically correct English sentence.
This grammatically is sentence not English correct a.
The second line is difficult to parse because it doesnât follow the rules of English. Similarly, if you enter a bad Python instruction, Python wonât be able to understand it and will display a SyntaxError error message, as shown here:
```
>>> 5 +
File "<python-input-0>", line 1
5 +
^
SyntaxError: invalid syntax
>>> 42 + 5 + * 2
File "<python-input-0>", line 1
42 + 5 + * 2
^
SyntaxError: invalid syntax
```
You can always test whether an instruction works by entering it into the interactive shell. Donât worry about breaking the computer; the worst that could happen is that Python responds with an error message. Professional software developers get error messages all the time while writing code.
### The Integer, Floating-Point, and String Data Types
Remember that expressions are just values combined with operators, and they always evaluate to a single value. A *[data type](https://docs.python.org/3/library/stdtypes.html)* is a category for values, and every value belongs to exactly one data type. The most common data types in Python are listed in Table 1-2. The values \-2 and 30, for example, are said to be *[integer](https://docs.python.org/3/library/stdtypes.html#numeric-types-int-float-complex)* values. The integer (or *int*) data type indicates values that are whole numbers. Numbers with a decimal point, such as 3\.14, are called *[floating-point numbers](https://docs.python.org/3/library/stdtypes.html#numeric-types-int-float-complex)* (or *floats*). Note that even though the value 42 is an integer, the value 42\.0 would be a floating-point number. Programmers often use *number* to refer to ints and floats collectively, although *number* itself is not a Python data type.
One subtle detail about Python is that any math performed using an int and a float results in a float, not an int. While 3 \+ 4 evaluates to the integer 7, the expression 3 \+ 4\.0 evaluates to the floating-point number 7\.0. Any division between two integers with the / division operator results in a float as well. For example, 16 / 4 evaluates to 4\.0 and not 4. Most of the time, this information doesnât matter for your program, but knowing it will explain why your numbers may suddenly gain a decimal point.
| Data type | Examples |
|---|---|
| Integer (int) | \-2, \-1, 0, 1, 2, 3, 4, 5 |
| Floating-point number (float) | \-1.25, \-1.0, \-0.5, 0\.0, 0\.5, 1\.0, 1\.25 |
| String (str) | 'a', 'aa', 'aaa', 'Hello!', '11 cats', '5' |
Python programs can also have text values called *[strings](https://docs.python.org/3/library/stdtypes.html#text-sequence-type-str)*, or *strs* (pronounced âstirsâ). Always surround your string in single-quote (') characters (as in 'Hello' or 'Goodbye cruel world!') so that Python knows where the string begins and ends. You can even have a string with no characters in it, '', called a *blank string* or an *empty string*. Strings are explained in greater detail in Chapter 8.
You may see the error message SyntaxError: unterminated string literal, as in this example:
```
>>> 'Hello, world!
SyntaxError: unterminated string literal (detected at line 1)
```
This error means you probably forgot the final single-quote character at the end of the string.
### String Concatenation and Replication
The meaning of an operator may change based on the data types of the values next to it. For example, \+ is the addition operator when it operates on two integers or floating-point values. However, when \+ is used to combine two string values, it joins the strings as the *string concatenation* operator. Enter the following into the interactive shell:
```
>>> 'Alice' + 'Bob'
'AliceBob'
```
The expression evaluates down to a single, new string value that combines the text of the two strings. However, if you try to use the \+ operator on a string and an integer value, Python wonât know how to handle this and will display an error message:
```
>>> 'Alice' + 42
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' + 42
TypeError: can only concatenate str (not "int") to str
```
The error message can only concatenate str (not "int") to str means that Python thought you were trying to concatenate an integer to the string 'Alice'. Your code will have to explicitly convert the integer to a string because Python cannot do this automatically. (Iâll explain how to convert between data types in âDissecting the Programâ on page 14, where we talk about the [str()](https://docs.python.org/3/library/functions.html#func-str), [int()](https://docs.python.org/3/library/functions.html#int), and [float()](https://docs.python.org/3/library/functions.html#float) functions.)
The \* operator multiplies two integer or floating-point values. But when the \* operator is used on one string value and one integer value, it becomes the *string replication* operator. Enter a string multiplied by a number into the interactive shell to see this in action:
```
>>> 'Alice' * 5
'AliceAliceAliceAliceAlice'
```
The expression evaluates down to a single string value that repeats the original string a number of times equal to the integer value. String replication is a useful trick, but itâs not used as often as string concatenation.
The \* operator can only be used with two numeric values (for multiplication), or one string value and one integer value (for string replication). Otherwise, Python will just display an error message, such as the following:
```
>>> 'Alice' * 'Bob'
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' * 'Bob'
TypeError: can't multiply sequence by non-int of type 'str'
>>> 'Alice' * 5.0
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' * 5.0
TypeError: can't multiply sequence by non-int of type 'float'
```
It makes sense that Python wouldnât understand these expressions: you canât multiply two words, and itâs hard to replicate an arbitrary string a fractional number of times.
Expressions, data types, and operators may seem abstract to you right now, but as you learn more about these concepts, youâll be able to create increasingly sophisticated programs that do math on data pulled from spreadsheets, websites, the output of other programs, and other places.
### Storing Values in Variables
A *variable* is like a box in the computerâs memory where you can store a single value. If you want to use the result of an evaluated expression later in your program, you can save it inside a variable.
#### Assignment Statements
Youâll store values in variables with an *assignment statement*. An assignment statement consists of a variable name, an equal sign (called the *assignment operator*), and the value to be stored. If you enter the assignment statement spam \= 42, a variable named spam will have the integer value 42 stored in it.
You can think of a variable as a labeled box that a value is placed in, but Chapter 6 explains how a name tag attached to the value might be a better metaphor. Both are shown in Figure 1-1.
Figure 1-1: The code spam = 42 is like telling the program, âThe variable spam now has the integer value 42 in it.â
For example, enter the following into the interactive shell:
```
âś >>> spam = 40
>>> spam
40
>>> eggs = 2
⡠>>> spam + eggs
42
>>> spam + eggs + spam
82
⸠>>> spam = spam + 2
>>> spam
42
```
A variable is *initialized* (or created) the first time a value is stored in it âś. After that, you can use it in expressions with other variables and values âˇ. When a variable is assigned a new value â¸, the old value is forgotten, which is why spam evaluated to 42 instead of 40 at the end of the example. This is called *overwriting* the variable. Enter the following code into the interactive shell to try overwriting a string:
```
>>> spam = 'Hello'
>>> spam
'Hello'
>>> spam = 'Goodbye'
>>> spam
'Goodbye'
```
Just like the box in Figure 1-1, the spam variable in Figure 1-2 stores 'Hello' until you replace the string with 'Goodbye'.
Figure 1-2: When a new value is assigned to a variable, the old one is forgotten.
You can also think of overwriting a variable as reassigning the name tag to a new value.
#### Variable Names
A good variable name describes the data it contains. Imagine that you moved to a new house and labeled all of your moving boxes as *Stuff*. Youâd never find anything! Most of this bookâs examples (and Pythonâs documentation) use generic variable names like spam, eggs, and bacon, which come from the Monty Python âSpamâ sketch. But in your programs, descriptive names will help make your code more readable.
Though you can name your variables almost anything, Python does have some naming restrictions. Your variable name must obey the following four rules:
- It canât have spaces.
- It can use only letters, numbers, and the underscore (\_) character.
- It canât begin with a number.
- It canât be a Python [keyword](https://docs.python.org/3/reference/lexical_analysis.html#keywords), such as if, for, return, or other keywords youâll learn in this book.
Table 1-3 shows examples of legal variable names.
| Valid variable names | Invalid variable names |
|---|---|
| current\_balance | current-balance (hyphens are not allowed) |
| currentBalance | current balance (spaces are not allowed) |
| account4 | 4account (canât begin with a number) |
| \_42 | 42 (can begin with an underscore but not a number) |
| TOTAL\_SUM | TOTAL\_\$UM (special characters like \$ are not allowed) |
| hello | 'hello' (special characters like ' are not allowed) |
Variable names are case-sensitive, meaning that spam, SPAM, Spam, and sPaM are four different variables. It is a Python convention to start your variables with a lowercase letter: spam instead of Spam.
CODE STYLE OPINIONS AND PEP 8
Previous editions of this book used camelCase instead of underscores to separate words in variable names; that is, variables lookedLikeThis instead of looking\_like\_this. The latter form is called snake\_case because the underscores between words look like little snakes (while the uppercase letters in camelCase look like the humps on a camel). Some experienced programmers may point out that the official Python code style document, [PEP 8](https://peps.python.org/pep-0008/), says that underscores should be used. I unapologetically prefer camelCase and point to the âA Foolish Consistency Is the Hobgoblin of Little Mindsâ section in PEP 8 itself as my defense:
Consistency with the style guide is important. But most importantly: know when to be inconsistentâsometimes the style guide just doesnât apply. When in doubt, use your best judgment.
The computer doesnât care which style you use, and PEP 8 is not a stone tablet of irrefutable commandments. It doesnât matter which style you use as long as you use the same style consistently. To prove this, Iâve rewritten the code in this book to use snake\_case because it truly doesnât matter either way.
### Your First Program
While the interactive shell is good for running Python instructions one at a time, to write entire Python programs youâll enter the instructions into the file editor. The *file editor* is similar to text editors such as Notepad and TextMate, but it has some features specifically for entering source code. To open a new file in Mu, click the **New** button on the top row.
The tab that appears should contain a cursor awaiting your input, but itâs different from the interactive shell, which runs Python instructions as soon as you press ENTER. The file editor lets you enter many instructions, save the file, and run the program. Hereâs how you can tell the difference between the two:
- The interactive shell will always be the one with the \>\>\> or In \[1\]: prompt.
- The file editor wonât have the \>\>\> or In \[1\]: prompt.
Now itâs time to create your first program! When the file editor window opens, enter the following into it:
```
# This program says hello and asks for my name.
print('Hello, world!')
print('What is your name?') # Ask for their name.
my_name = input('>')
print('It is good to meet you, ' + my_name)
print('The length of your name is:')
print(len(my_name))
print('What is your age?') # Ask for their age.
my_age = input('>')
print('You will be ' + str(int(my_age) + 1) + ' in a year.')
```
Once youâve entered your source code, save it so that you wonât have to retype it each time you start Mu. Click **Save**, enter **hello.py** in the File Name field, and then click **Save**.
You should save your programs every once in a while as you type them. That way, if the computer crashes or you accidentally exit Mu, you wonât lose the code. As a shortcut, you can press CTRL\-S on Windows and Linux or ďş\-S on macOS to save your file.
Once youâve saved, letâs run our program. Press the F5 key or click the **Run** button. Enter your name when your program asks for it. The programâs output in the interactive shell should look something like this:
```
Hello, world!
What is your name?
>Al
It is good to meet you, Al
The length of your name is:
2
What is your age?
>4
You will be 5 in a year.
>>>
```
When there are no more lines of code to execute, the Python program *terminates*; that is, it stops running. (You can also say that the Python program *exits*.) The Mu editor displays the \>\>\> interactive shell prompt after the program terminated, in case youâd like to enter some further Python code.
You can close the file editor by clicking the **X** on the fileâs tab, just like closing a browser tab. To reload a saved program, click **Load** from the menu. Do that now, and in the window that appears, choose **hello.py** and click the **Open** button. Your previously saved *hello.py* program should open in the file editor window.
You can view the step-by-step execution of a program using the Python Tutor visualization tool at *<http://pythontutor.com>*. Click the forward button to move through each step of the programâs execution. Youâll be able to see how the variablesâ values and the output change.
### Dissecting the Program
With your new program open in the file editor, letâs take a quick tour of the Python instructions it uses by looking at what each line of code does.
#### Comments
The following line is called a *comment*:
```
# This program says hello and asks for my name.
```
Python ignores comments, and you can use them to write notes or remind yourself what the code is trying to do. Any text for the rest of the line following a hash mark (\#) is part of a comment.
Sometimes programmers will put a \# in front of a line of code to temporarily remove it while testing a program. This is called *commenting out* code, and it can be useful when youâre trying to figure out why a program isnât working. You can remove the \# later when you are ready to put the line back in.
Python also ignores the blank line after the comment. You can add as many blank lines to your program as you want. This spacing can make your code easier to read, like paragraphs in a book.
#### The print() Function
The [print()](https://docs.python.org/3/library/functions.html#print) function displays the string value inside its parentheses on the screen:
```
print('Hello, world!')
print('What is your name?') # Ask for their name.
```
The line print('Hello, world!') means âPrint out the text in the string 'Hello, world!'.â When Python executes this line, you say that Python is *calling* the print() function and the string value is being *passed* to the function. A value that is passed to a function call is an *argument.* Notice that the quotes are not printed to the screen. They just mark where the string begins and ends; they are not part of the string valueâs text.
> NOTE
*You can also use this function to display a blank line on the screen; call print() with nothing in between the parentheses.*
When you write a function name, the opening and closing parentheses at the end identify it as the name of a function. This is why in this book, youâll see print() rather than print. Itâs a standard convention to have no spaces in between the function name and the opening parentheses, even though Python doesnât require this. Chapter 3 describes functions in more detail.
#### The input() Function
The [input()](https://docs.python.org/3/library/functions.html#input) function waits for the user to type some text on the keyboard and press ENTER:
```
my_name = input('>')
```
This function call evaluates to a string identical to the userâs text, and the rest of the code assigns the my\_name variable to this string value. The '\>' string passed to the function causes the \> prompt to appear, which serves as an indicator to the user that they are expected to enter something. Your programs donât have to pass a string to the input() function; if you call input(), the program will wait for the userâs text without displaying any prompt.
THE \> AND \>\>\> PROMPTS
You can pass any string to input() to change the prompt that appears when your program runs. Calling input('\>') puts an angle bracket \> on the screen as the prompt. This is different from the \>\>\> prompt that appears in the Python interactive shell. In this book, the \>\>\> prompt indicates a response to the Python interactive shell and the \> prompt indicates a response to a program running the input('\>') call. My choice of '\>' was arbitrary; you can use any prompt you want or no prompt at all.
You can think of the input() function call as an expression that evaluates to whatever string the user typed. If the user entered 'Al', the assignment statement would effectively be my\_name \= 'Al'.
If you call input() and see an error message, like NameError: name 'Al' is not defined, the problem is that youâre running the code with Python 2 instead of Python 3.
#### The Greeting Message
The following call to print() contains the expression 'It is good to meet you, ' \+ my\_name between the parentheses:
```
print('It is good to meet you, ' + my_name)
```
Remember that expressions can always evaluate to a single value. If 'Al' is the value stored in my\_name, then this expression evaluates to 'It is good to meet you, Al'. This single string value is then passed to print(), which prints it on the screen.
#### The len() Function
You can pass the [len()](https://docs.python.org/3/library/functions.html#len) function a string value (or a variable containing a string), and the function evaluates to the integer value of the number of characters in that string:
```
print('The length of your name is:')
print(len(my_name))
```
Enter the following into the interactive shell to try this:
```
>>> len('hello')
5
>>> len('My very energetic monster just scarfed nachos.')
46
>>> len('')
0
```
Just like in those examples, len(my\_name) evaluates to an integer. We say that the len() function call *returns* or *outputs* this integer value, and the value is the function callâs *return value*. It is then passed to print() to be displayed on the screen. The print() function allows you to pass it either integer values or string values, but notice the error that shows up when you enter the following into the interactive shell:
```
>>> print('I am ' + 29 + ' years old.')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
print('I am ' + 29 + ' years old.')
TypeError: can only concatenate str (not "int") to str
```
The print() function isnât causing that error; rather, itâs the expression you tried to pass to print(). Youâll get the same error message if you type the expression into the interactive shell on its own:
```
>>> 'I am ' + 29 + ' years old.'
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'I am ' + 29 + ' years old.'
TypeError: can only concatenate str (not "int") to str
```
Python gives an error because the \+ operator can be used only to add two numbers together or to concatenate two strings. You canât add an integer to a string because this is not allowed in Python. You can fix this by using a string version of the integer instead, as explained in the next section.
#### The str(), int(), and float() Functions
If you want to concatenate an integer such as 29 with a string to pass to print(), youâll need to get the value '29', which is the string form of 29. The str() function can be passed an integer value and will return a string value version of the integer, as follows:
```
>>> str(29)
'29'
>>> print('I am ' + str(29) + ' years old.')
I am 29 years old.
```
Because str(29) evaluates to '29', the expression 'I am ' \+ str(29) \+ ' years old.' evaluates to 'I am ' \+ '29' \+ ' years old.', which in turn evaluates to 'I am 29 years old.' This is the string value that is passed to the print() function.
The str(), int(), and float() functions will evaluate to the string, integer, and floating-point forms of the value you pass, respectively. Try converting some values in the interactive shell with these functions, and watch what happens:
```
>>> str(0)
'0'
>>> str(-3.14)
'-3.14'
>>> int('42')
42
>>> int('-99')
-99
>>> int(1.25)
1
>>> int(1.99)
1
>>> float('3.14')
3.14
>>> float(10)
10.0
```
The previous examples call the str(), int(), and float() functions and pass them values of the other data types to obtain a string, integer, or floating-point form of those values.
The str() function is handy when you have an integer or float that you want to concatenate to a string. The int() function is also helpful if you have a number as a string value that you want to use in some mathematics. For example, the input() function always returns a string, even if the user enters a number. Enter spam \= input('\>') into the interactive shell, then enter 101 when it waits for your text:
```
>>> spam = input('>')
>101
>>> spam
'101'
```
The value stored inside spam isnât the integer 101 but the string '101'. If you want to do math using the value in spam, use the int() function to get its integer form and then store this as the variableâs new value. If spam is the string '101', then the expression int(spam) will evaluate to the integer value 101, and the assignment statement spam \= int(spam) will be equivalent to spam \= 101:
```
>>> spam = int(spam)
>>> spam
101
```
Now you should be able to treat the spam variable as an integer instead of a string:
```
>>> spam * 10 / 5
202.0
```
Note that if you pass a value to int() that it cannot evaluate as an integer, Python will display an error message:
```
>>> int('99.99')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
int('99.99')
ValueError: invalid literal for int() with base 10: '99.99'
>>> int('twelve')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
int('twelve')
ValueError: invalid literal for int() with base 10: 'twelve'
```
The int() function is also useful if you need to round a floating-point number down:
```
>>> int(7.7)
7
>>> int(7.7) + 1
8
```
You used the int() and str() functions in the last three lines of your program to get a value of the appropriate data type for the code:
```
print('What is your age?') # Ask for their age.
my_age = input('>')
print('You will be ' + str(int(my_age) + 1) + ' in a year.')
```
The my\_age variable contains the value returned from input(). Because the input() function always returns a string (even if the user entered a number), you can use the int(my\_age) code to return an integer value of the string in my\_age. This integer value is then added to 1 in the expression int(my\_age) \+ 1.
TEXT AND NUMBER EQUIVALENCE
Although the string value of a number is considered a completely different value from the integer or floating-point version, an integer can be equal to a floating point:
```
>>> 42 == '42'
False
>>> 42 == 42.0
True
>>> 42.0 == 0042.000
True
```
Python makes this distinction because strings are text, while integers and floats are numbers.
The result of this addition is passed to the str() function: str(int(my \_age) \+ 1\). The string value returned is then concatenated with the strings 'You will be ' and ' in a year.' to evaluate to one large string value. This large string is finally passed to print() to be displayed on the screen.
Letâs say the user enters the string '4' for my\_age. The evaluation steps would look something like the following:
Description
The string '4' is converted to an integer, so you can add 1 to it. The result is 5. The str() function converts the result back to a string, so you can concatenate it with the second string, 'in a year.', to create the final message.
#### The type() Function
Integer, floating-point, and string arenât the only data types in Python. As you continue to learn about programming, you may come across values of other data types. You can always pass these to the [type()](https://docs.python.org/3/library/functions.html#type) function to determine what type they are. For example, enter the following into the interactive shell:
```
>>> type(42)
<class 'int'>
>>> type(42.0)
<class 'float'>
>>> type('forty two')
<class 'str'>
>>> name = 'Zophie'
>>> type(name) # The name variable has a value of the string type.
<class 'str'>
>>> type(len(name)) # The len() function returns integer values.
<class 'int'>
```
Not only can you pass any value to type(), but (as with any function call) you can also pass it any variable or expression to determine the data type of the value that it evaluates to. The type() function itself returns values, but the angle brackets mean they are not syntactically valid Python code; you cannot run code like spam \= \<class 'str'\>.
#### The round() and abs() Functions
Letâs learn about two more Python functions that, like the len() function, take an argument and return a value. The [round()](https://docs.python.org/3/library/functions.html#round) function accepts a float value and returns the nearest integer. Enter the following into the interactive shell:
```
>>> round(3.14)
3
>>> round(7.7)
8
>>> round(-2.2)
-2
```
The round() function also accepts an optional second argument specifying how many decimal places it should round. Enter the following into the interactive shell:
```
>>> round(3.14, 1)
3.1
>>> round(7.7777, 3)
7.778
```
The behavior for rounding half numbers is a bit odd. The function call round(3.5) rounds up to 4, while round(2.5) rounds down to 2. For halfway numbers that end with .5, the number is rounded to the nearest even integer. This is called *bankerâs rounding*.
The [abs()](https://docs.python.org/3/library/functions.html#abs) function returns the absolute value of the number argument. In mathematics, this is defined as the distance from 0, but I find it easier to think of it as the positive form of the number. Enter the following into the interactive shell:
```
>>> abs(25)
25
>>> abs(-25)
25
>>> abs(-3.14)
3.14
>>> abs(0)
0
```
Python comes with several different functions that youâll learn about in this book. This section demonstrates how you can experiment with them in the interactive shell to see how they behave with different inputs. This is a common technique for practicing the new code that you learn.
### How Computers Store Data with Binary Numbers
Thatâs enough Python code for now. At this point, you might think that programming seems almost magical. How does the computer know to transform 2 \+ 2 into 4? The answer is too complicated for this book, but I can explain part of whatâs going on behind the scenes by discussing what binary numbers (numbers that have only the digits 1 and 0) have to do with computing.
Hacking in movies often involves streams of 1s and 0s flowing across the screen. This looks mysterious and impressive, but what do these 1s and 0s actually mean? The answer is that binary is the simplest number system, and it can be implemented with inexpensive components for computer hardware. Binary, also called the *base-2 number system*, can represent all of the same numbers that our more familiar base-10 *decimal number system* can. Decimal has 10 digits, 0 through 9. Table 1-4 shows the first 27 integers in decimal and binary.
| Decimal | Binary | Decimal | Binary | Decimal | Binary |
|---|---|---|---|---|---|
| 0 | 0 | 9 | 1001 | 18 | 10010 |
| 1 | 1 | 10 | 1010 | 19 | 10011 |
| 2 | 10 | 11 | 1011 | 20 | 10100 |
| 3 | 11 | 12 | 1100 | 21 | 10101 |
| 4 | 100 | 13 | 1101 | 22 | 10110 |
| 5 | 101 | 14 | 1110 | 23 | 10111 |
| 6 | 110 | 15 | 1111 | 24 | 11000 |
| 7 | 111 | 16 | 10000 | 25 | 11001 |
| 8 | 1000 | 17 | 10001 | 26 | 11010 |
Think of these number systems as a mechanical odometer, like in Figure 1-3. When you reach the last digit, they each reset to 0 while incrementing the value of the next digit over. In decimal, the last digit is 9, and in binary, the last digit is 1. Thatâs why the decimal number after 9 is 10 and the decimal number after 999 is 1000. Similarly, the binary number after 1 is 10 and the binary number after 111 is 1000. However, *10* in binary doesnât represent the same quantity as *ten* in decimal; rather, it represents *two*. And *1000* in binary doesnât mean *one thousand* in decimal, but rather *eight*. You can view an interactive binary and decimal odometer at *[https://inventwithpython.com/odometer](https://inventwithpython.com/odometer)*.
Figure 1-3: A mechanical odometer in decimal (left) and in binary (right)
Representing binary numbers with computer hardware is simpler than representing decimal numbers because there are only two states to represent. For example, Blu-ray discs and DVDs have smooth *lands* and indented *pits* etched on their surface that will or wonât reflect the disc playerâs laser, respectively. Circuits can have electric current flowing through them or no electric current. These various hardware standards all have ways of representing two different states. On the other hand, itâd be expensive to create high-quality electronic components that are sensitive enough to detect the difference between 10 different voltage levels with reliable accuracy. Itâs more economical to use simple components, and two binary states are as simple as you can get.
These binary digits are called *bits* for short. A single bit can represent two numbers, and 8 bits (or 1 *byte*) can represent 28, or 256, numbers, ranging from 0 to 255 in decimal or 0 to 11111111 in binary. This is similar to how a single decimal digit can represent 10 numbers (0 to 9), while an eight-digit decimal number can represent 108 or 100,000,000 numbers (0 to 99,999,999). Files on your computer are measured in how many bytes they take up:
- A kilobyte (KB) is 210 or 1,024 bytes.
- A megabyte (MB) is 220 or 1,048,576 bytes (or 1,024KB).
- A gigabyte (GB) is 230 or 1,073,741,824 bytes (or 1,024MB).
- A terabyte (TB) is 240 or 1,099,511,627,776 bytes (or 1,024GB).
The text of Shakespeareâs *Romeo and Juliet* is about 135KB. A high-resolution photo is about 2MB to 5MB. A movie can be anywhere from 1GB to 50GB, depending on picture quality and movie length. However, hard drive and flash memory manufacturers blatantly lie about what these terms mean. For example, by calling a TB 1,000,000,000,000 bytes instead of 1,099,511,627,776 bytes, they can advertise a 9.09TB hard drive as 10TB.
The 1s and 0s of binary can represent not only any integer but also any form of data. Instead of 0 to 255, a byte can represent the numbers â128 to 127 using a system called *twoâs complement*. Fractional floating-point numbers can be represented in binary using a system called *IEEE-754*.
Text can be stored on computers as binary numbers by assigning each letter, punctuation mark, or symbol a unique number. A system for representing text as numbers is called an *encoding*. The most popular encoding for text is UTF-8. In UTF-8, a capital letter *A* is represented by the decimal number 65 (or 01000001 as an 8-bit binary number), a *?* (question mark) is represented by the number 63, and the numeral character *7* is represented by the number 55. The string 'Hello' is stored as the numbers 72, 101, 108, 108, and 111. When stored in a computer, âHelloâ appears as a stream of bits: 0100100001100101011011000110110001101111.
Wow! Just like in those hacker movies\!
Engineers need to invent a way to encode each form of data as numbers. Photos and images can be broken up into a two-dimensional grid of colored squares called *pixels*. Each pixel can use three bytes to represent how much red, green, and blue color it contains. (Chapter 21 covers image data in more detail.) But for a short example, the numbers 255, 0, and 255 could represent a pixel with the maximum amount of red and blue but zero green, resulting in a purple pixel.
Sound is made up of waves of compressed air that reach our ears, which our brains interpret as audio sensation. We can graph the intensity and frequency of these waves over time. The numbers on this graph can then be converted to binary numbers and stored on a computer, which later control speakers to reproduce the sound. This is a simplification of how computer audio works, but describes how numbers can represent Beethovenâs Symphony No. 5.
The data for several images combines with audio data to store videos. All forms of information can be encoded into binary numbers. There is, of course, a great deal more detail to it than this, but this is how 1s and 0s represent the wide variety of data in our information age.
### Summary
You can compute expressions with a calculator or enter string concatenations with a word processor. You can even do string replication easily by copying and pasting text. But expressions, and their component valuesâoperators, variables, and function callsâare the basic building blocks that make up programs. Once you know how to handle these elements, you will be able to instruct Python to operate on large amounts of data for you.
Youâll find it helpful to remember the different types of operators (\+, \-, \*, /, //, %, and \*\* for math operations, and \+ and \* for string operations) and the three data types (integers, floating-point numbers, and strings) introduced in this chapter.
I introduced a few different functions as well. The print() and input() functions handle simple text output (to the screen) and input (from the keyboard). The len() function takes a string and evaluates to an int of the number of characters in the string. The str(), int(), and float() functions will evaluate to the string, integer, or floating-point number form of the value they are passed. The round() function returns the rounded integer, and the abs() function returns the absolute value of the arguments.
In the next chapter, youâll learn how to tell Python to make intelligent decisions about what code to run, what code to skip, and what code to repeat based on the values it has. This is known as *flow control*, and it allows you to write programs that make intelligent decisions.
### Practice Questions
1\. Which of the following are operators, and which are values?
```
*
'hello'
-88.8
-
/
+
5
```
2\. Which of the following is a variable, and which is a string?
```
spam
'spam'
```
3\. Name three data types.
4\. What is an expression made up of? What do all expressions do?
5\. This chapter introduced assignment statements, like spam \= 10. What is the difference between an expression and a statement?
6\. What does the variable bacon contain after the following code runs?
```
bacon = 20
bacon + 1
```
7\. What should the following two expressions evaluate to?
```
'spam' + 'spamspam'
'spam' * 3
```
8\. Why is eggs a valid variable name while 100 is invalid?
9\. What three functions can be used to get the integer, floating-point number, or string version of a value?
10\. Why does this expression cause an error? How can you fix it?
```
'I eat ' + 99 + ' burritos.'
```
**Extra credit:** Search online for the Python documentation for the len() function. It will be on a web page titled âBuilt-in Functions.â Skim the list of other functions Python has, look up what the bin() and hex() functions do, and experiment with them in the interactive shell.
(close)
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[...and other books as well\!](https://inventwithpython.com/) Or register for the [online video course.](https://inventwithpython.com/automateudemy) You can also [donate to support the author directly.](https://inventwithpython.com/#donate) |
| Readable Markdown | 
The Python programming language has a wide range of syntactical constructions, standard library functions, and interactive development environment features. Fortunately, you can ignore most of that; you just need to learn enough to write some handy little programs.
To accomplish this, however, youâll have to master some programming concepts. Like a wizard in training, you might think these concepts seem tedious, but with some practice, theyâll enable you to command your computer like a magic wand and perform incredible feats.
This chapter has a few examples that encourage you to enter code into the *[interactive shell](https://docs.python.org/3/tutorial/interpreter.html#interactive-mode)*, also called the *read-evaluate-print-loop (REPL)*, which lets you run (or *execute*) Python instructions one at a time and instantly shows you the results. Using the interactive shell is great for learning what basic Python instructions do, so give it a try as you follow along. Youâll remember the things you do much better than the things you only read.
### Entering Expressions into the Interactive Shell
You can run the interactive shell by launching the Mu editor. This bookâs introduction provides setup instructions for downloading and installing it. On Windows, open the Start menu, enter **Mu**, and open the Mu app. On macOS, open your Applications folder and double-click **Mu**. Click the **New** button and save an empty file as *blank.py*. When you run this blank file by clicking the **Run** button or pressing F5, it will open the interactive shell, which will open as a new pane at the bottom of the Mu editorâs window. You should see a \>\>\> prompt in the interactive shell.
You can also run the interactive shell from the command line Terminal (on macOS and Linux) or Windows Terminal (on Windows, where you could also use the older Command Prompt application). After opening these command line windows, enter python (on Windows) or python3 (on macOS and Linux). Youâll see the same \>\>\> prompt for the interactive shell. If you want to run a program, run python or python3 followed by the name of the programâs *.py* file, such as python blank.py. Be sure you donât run python on macOSâs Terminal, as this may launch the older, backward-incompatible Python 2.7 version on certain versions of macOS. You may even see a message saying WARNING: Python 2.7 is not recommended. Exit the 2.7 interactive shell and run python3 instead.
Enter 2 \+ 2 at the prompt to have Python do some simple math. The Mu window should now look like this:
```
>>> 2 + 2
4
>>>
```
In Python, 2 \+ 2 is called an *expression*, which is the most basic kind of programming instruction in the language. Expressions consist of *values* (such as 2) and *operators* (such as \+), and they can always *evaluate* (that is, reduce) down to a single value. That means you can use expressions anywhere in Python code that you could also use a value.
In the previous example, 2 \+ 2 is evaluated down to a single value, 4. A single value with no operators is also considered an expression, though it evaluates only to itself, as shown here:
```
>>> 2
2
```
The Mu editor has a REPL button that shows an interactive shell with a prompt that looks like In \[1\]:. The popular Jupyter Notebook editor uses this kind of interactive shell. You can use this interactive shell the same way as the normal Python interactive shell with the \>\>\> prompt. REPLs are not unique to Python; many programming languages also offer REPLs so that you can experiment with their code.
Programming involves some math operations you might not be familiar with:
- Exponentiation (or *to the power of*) is multiplying a number by itself repeatedly, just like multiplication is adding a number to itself repeatedly. For example, *two to the power of four* (or *two to the fourth power*), written as or 24 or 2 \*\* 4, is the number two multiplied by itself four times: 24 = 2 Ă 2 Ă 2 Ă 2 = 16.
- Modular arithmetic is similar to the remainder result of division. For example, 14 % 4 evaluates to 2 because 14 divided by 4 is 3 with remainder 2. Even though Pythonâs modulo operator is %, modular arithmetic has nothing to do with percentages.
- Integer division is the same as regular division except the result is rounded down. For example, 25 / 8 is 3\.125 but 25 // 8 is 3, and 29 / 10 is 2\.9 but 29 // 10 is 2.
You can use plenty of other operators in Python expressions too. For example, Table 1-1 lists all the math operators in Python.
| Operator | Operation | Example | Evaluates to ... |
|---|---|---|---|
| \*\* | Exponentiation | 2 \*\* 3 | 8 |
| % | Modulus/remainder | 22 % 8 | 6 |
| // | Integer division | 22 // 8 | 2 |
| / | Division | 22 / 8 | 2\.75 |
| \* | Multiplication | 3 \* 5 | 15 |
| \- | Subtraction | 5 - 2 | 3 |
| \+ | Addition | 2 \+ 2 | 4 |
The *order of operations* (also called *precedence*) of Python math operators is similar to that of mathematics. The \*\* operator is evaluated first; the \*, /, //, and % operators are evaluated next, from left to right; and the \+ and \- operators are evaluated last (also from left to right). You can use parentheses to override the usual precedence if you need to. Whitespace in between the operators and values doesnât matter in Python, except for the indentation at the beginning of the line. But the *convention*, or unofficial rule, is to have a single space in between operators and values. Enter the following expressions into the interactive shell:
```
>>> 2 + 3 * 6
20
>>> (2 + 3) * 6
30
>>> 48565878 * 578453
28093077826734
>>> 2 ** 8
256
>>> 23 / 7
3.2857142857142856
>>> 23 // 7
3
>>> 23 % 7
2
>>> 2 + 2
4
>>> (5 - 1) * ((7 + 1) / (3 - 1))
16.0
```
In each case, you as the programmer must enter the expression, but Python does the hard part of evaluating it. Python will keep evaluating parts of the expression until it becomes a single value:
These rules for gether to form expressions are a fundamental part of Python as a programming language, just like the grammar rules that help us communicate. Hereâs an example:
This is a grammatically correct English sentence.
This grammatically is sentence not English correct a.
The second line is difficult to parse because it doesnât follow the rules of English. Similarly, if you enter a bad Python instruction, Python wonât be able to understand it and will display a SyntaxError error message, as shown here:
```
>>> 5 +
File "<python-input-0>", line 1
5 +
^
SyntaxError: invalid syntax
>>> 42 + 5 + * 2
File "<python-input-0>", line 1
42 + 5 + * 2
^
SyntaxError: invalid syntax
```
You can always test whether an instruction works by entering it into the interactive shell. Donât worry about breaking the computer; the worst that could happen is that Python responds with an error message. Professional software developers get error messages all the time while writing code.
### The Integer, Floating-Point, and String Data Types
Remember that expressions are just values combined with operators, and they always evaluate to a single value. A *[data type](https://docs.python.org/3/library/stdtypes.html)* is a category for values, and every value belongs to exactly one data type. The most common data types in Python are listed in Table 1-2. The values \-2 and 30, for example, are said to be *[integer](https://docs.python.org/3/library/stdtypes.html#numeric-types-int-float-complex)* values. The integer (or *int*) data type indicates values that are whole numbers. Numbers with a decimal point, such as 3\.14, are called *[floating-point numbers](https://docs.python.org/3/library/stdtypes.html#numeric-types-int-float-complex)* (or *floats*). Note that even though the value 42 is an integer, the value 42\.0 would be a floating-point number. Programmers often use *number* to refer to ints and floats collectively, although *number* itself is not a Python data type.
One subtle detail about Python is that any math performed using an int and a float results in a float, not an int. While 3 \+ 4 evaluates to the integer 7, the expression 3 \+ 4\.0 evaluates to the floating-point number 7\.0. Any division between two integers with the / division operator results in a float as well. For example, 16 / 4 evaluates to 4\.0 and not 4. Most of the time, this information doesnât matter for your program, but knowing it will explain why your numbers may suddenly gain a decimal point.
| Data type | Examples |
|---|---|
| Integer (int) | \-2, \-1, 0, 1, 2, 3, 4, 5 |
| Floating-point number (float) | \-1.25, \-1.0, \-0.5, 0\.0, 0\.5, 1\.0, 1\.25 |
| String (str) | 'a', 'aa', 'aaa', 'Hello!', '11 cats', '5' |
Python programs can also have text values called *[strings](https://docs.python.org/3/library/stdtypes.html#text-sequence-type-str)*, or *strs* (pronounced âstirsâ). Always surround your string in single-quote (') characters (as in 'Hello' or 'Goodbye cruel world!') so that Python knows where the string begins and ends. You can even have a string with no characters in it, '', called a *blank string* or an *empty string*. Strings are explained in greater detail in Chapter 8.
You may see the error message SyntaxError: unterminated string literal, as in this example:
```
>>> 'Hello, world!
SyntaxError: unterminated string literal (detected at line 1)
```
This error means you probably forgot the final single-quote character at the end of the string.
### String Concatenation and Replication
The meaning of an operator may change based on the data types of the values next to it. For example, \+ is the addition operator when it operates on two integers or floating-point values. However, when \+ is used to combine two string values, it joins the strings as the *string concatenation* operator. Enter the following into the interactive shell:
```
>>> 'Alice' + 'Bob'
'AliceBob'
```
The expression evaluates down to a single, new string value that combines the text of the two strings. However, if you try to use the \+ operator on a string and an integer value, Python wonât know how to handle this and will display an error message:
```
>>> 'Alice' + 42
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' + 42
TypeError: can only concatenate str (not "int") to str
```
The error message can only concatenate str (not "int") to str means that Python thought you were trying to concatenate an integer to the string 'Alice'. Your code will have to explicitly convert the integer to a string because Python cannot do this automatically. (Iâll explain how to convert between data types in âDissecting the Programâ on page 14, where we talk about the [str()](https://docs.python.org/3/library/functions.html#func-str), [int()](https://docs.python.org/3/library/functions.html#int), and [float()](https://docs.python.org/3/library/functions.html#float) functions.)
The \* operator multiplies two integer or floating-point values. But when the \* operator is used on one string value and one integer value, it becomes the *string replication* operator. Enter a string multiplied by a number into the interactive shell to see this in action:
```
>>> 'Alice' * 5
'AliceAliceAliceAliceAlice'
```
The expression evaluates down to a single string value that repeats the original string a number of times equal to the integer value. String replication is a useful trick, but itâs not used as often as string concatenation.
The \* operator can only be used with two numeric values (for multiplication), or one string value and one integer value (for string replication). Otherwise, Python will just display an error message, such as the following:
```
>>> 'Alice' * 'Bob'
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' * 'Bob'
TypeError: can't multiply sequence by non-int of type 'str'
>>> 'Alice' * 5.0
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'Alice' * 5.0
TypeError: can't multiply sequence by non-int of type 'float'
```
It makes sense that Python wouldnât understand these expressions: you canât multiply two words, and itâs hard to replicate an arbitrary string a fractional number of times.
Expressions, data types, and operators may seem abstract to you right now, but as you learn more about these concepts, youâll be able to create increasingly sophisticated programs that do math on data pulled from spreadsheets, websites, the output of other programs, and other places.
### Storing Values in Variables
A *variable* is like a box in the computerâs memory where you can store a single value. If you want to use the result of an evaluated expression later in your program, you can save it inside a variable.
#### Assignment Statements
Youâll store values in variables with an *assignment statement*. An assignment statement consists of a variable name, an equal sign (called the *assignment operator*), and the value to be stored. If you enter the assignment statement spam \= 42, a variable named spam will have the integer value 42 stored in it.
You can think of a variable as a labeled box that a value is placed in, but Chapter 6 explains how a name tag attached to the value might be a better metaphor. Both are shown in Figure 1-1.
Figure 1-1: The code spam = 42 is like telling the program, âThe variable spam now has the integer value 42 in it.â
For example, enter the following into the interactive shell:
```
âś >>> spam = 40
>>> spam
40
>>> eggs = 2
⡠>>> spam + eggs
42
>>> spam + eggs + spam
82
⸠>>> spam = spam + 2
>>> spam
42
```
A variable is *initialized* (or created) the first time a value is stored in it âś. After that, you can use it in expressions with other variables and values âˇ. When a variable is assigned a new value â¸, the old value is forgotten, which is why spam evaluated to 42 instead of 40 at the end of the example. This is called *overwriting* the variable. Enter the following code into the interactive shell to try overwriting a string:
```
>>> spam = 'Hello'
>>> spam
'Hello'
>>> spam = 'Goodbye'
>>> spam
'Goodbye'
```
Just like the box in Figure 1-1, the spam variable in Figure 1-2 stores 'Hello' until you replace the string with 'Goodbye'.
Figure 1-2: When a new value is assigned to a variable, the old one is forgotten.
You can also think of overwriting a variable as reassigning the name tag to a new value.
#### Variable Names
A good variable name describes the data it contains. Imagine that you moved to a new house and labeled all of your moving boxes as *Stuff*. Youâd never find anything! Most of this bookâs examples (and Pythonâs documentation) use generic variable names like spam, eggs, and bacon, which come from the Monty Python âSpamâ sketch. But in your programs, descriptive names will help make your code more readable.
Though you can name your variables almost anything, Python does have some naming restrictions. Your variable name must obey the following four rules:
- It canât have spaces.
- It can use only letters, numbers, and the underscore (\_) character.
- It canât begin with a number.
- It canât be a Python [keyword](https://docs.python.org/3/reference/lexical_analysis.html#keywords), such as if, for, return, or other keywords youâll learn in this book.
Table 1-3 shows examples of legal variable names.
| Valid variable names | Invalid variable names |
|---|---|
| current\_balance | current-balance (hyphens are not allowed) |
| currentBalance | current balance (spaces are not allowed) |
| account4 | 4account (canât begin with a number) |
| \_42 | 42 (can begin with an underscore but not a number) |
| TOTAL\_SUM | TOTAL\_\$UM (special characters like \$ are not allowed) |
| hello | 'hello' (special characters like ' are not allowed) |
Variable names are case-sensitive, meaning that spam, SPAM, Spam, and sPaM are four different variables. It is a Python convention to start your variables with a lowercase letter: spam instead of Spam.
### Your First Program
While the interactive shell is good for running Python instructions one at a time, to write entire Python programs youâll enter the instructions into the file editor. The *file editor* is similar to text editors such as Notepad and TextMate, but it has some features specifically for entering source code. To open a new file in Mu, click the **New** button on the top row.
The tab that appears should contain a cursor awaiting your input, but itâs different from the interactive shell, which runs Python instructions as soon as you press ENTER. The file editor lets you enter many instructions, save the file, and run the program. Hereâs how you can tell the difference between the two:
- The interactive shell will always be the one with the \>\>\> or In \[1\]: prompt.
- The file editor wonât have the \>\>\> or In \[1\]: prompt.
Now itâs time to create your first program! When the file editor window opens, enter the following into it:
```
# This program says hello and asks for my name.
print('Hello, world!')
print('What is your name?') # Ask for their name.
my_name = input('>')
print('It is good to meet you, ' + my_name)
print('The length of your name is:')
print(len(my_name))
print('What is your age?') # Ask for their age.
my_age = input('>')
print('You will be ' + str(int(my_age) + 1) + ' in a year.')
```
Once youâve entered your source code, save it so that you wonât have to retype it each time you start Mu. Click **Save**, enter **hello.py** in the File Name field, and then click **Save**.
You should save your programs every once in a while as you type them. That way, if the computer crashes or you accidentally exit Mu, you wonât lose the code. As a shortcut, you can press CTRL\-S on Windows and Linux or ďş\-S on macOS to save your file.
Once youâve saved, letâs run our program. Press the F5 key or click the **Run** button. Enter your name when your program asks for it. The programâs output in the interactive shell should look something like this:
```
Hello, world!
What is your name?
>Al
It is good to meet you, Al
The length of your name is:
2
What is your age?
>4
You will be 5 in a year.
>>>
```
When there are no more lines of code to execute, the Python program *terminates*; that is, it stops running. (You can also say that the Python program *exits*.) The Mu editor displays the \>\>\> interactive shell prompt after the program terminated, in case youâd like to enter some further Python code.
You can close the file editor by clicking the **X** on the fileâs tab, just like closing a browser tab. To reload a saved program, click **Load** from the menu. Do that now, and in the window that appears, choose **hello.py** and click the **Open** button. Your previously saved *hello.py* program should open in the file editor window.
You can view the step-by-step execution of a program using the Python Tutor visualization tool at *[http://pythontutor.com](http://pythontutor.com/)*. Click the forward button to move through each step of the programâs execution. Youâll be able to see how the variablesâ values and the output change.
### Dissecting the Program
With your new program open in the file editor, letâs take a quick tour of the Python instructions it uses by looking at what each line of code does.
#### Comments
The following line is called a *comment*:
```
# This program says hello and asks for my name.
```
Python ignores comments, and you can use them to write notes or remind yourself what the code is trying to do. Any text for the rest of the line following a hash mark (\#) is part of a comment.
Sometimes programmers will put a \# in front of a line of code to temporarily remove it while testing a program. This is called *commenting out* code, and it can be useful when youâre trying to figure out why a program isnât working. You can remove the \# later when you are ready to put the line back in.
Python also ignores the blank line after the comment. You can add as many blank lines to your program as you want. This spacing can make your code easier to read, like paragraphs in a book.
#### The print() Function
The [print()](https://docs.python.org/3/library/functions.html#print) function displays the string value inside its parentheses on the screen:
```
print('Hello, world!')
print('What is your name?') # Ask for their name.
```
The line print('Hello, world!') means âPrint out the text in the string 'Hello, world!'.â When Python executes this line, you say that Python is *calling* the print() function and the string value is being *passed* to the function. A value that is passed to a function call is an *argument.* Notice that the quotes are not printed to the screen. They just mark where the string begins and ends; they are not part of the string valueâs text.
> NOTE
*You can also use this function to display a blank line on the screen; call print() with nothing in between the parentheses.*
When you write a function name, the opening and closing parentheses at the end identify it as the name of a function. This is why in this book, youâll see print() rather than print. Itâs a standard convention to have no spaces in between the function name and the opening parentheses, even though Python doesnât require this. Chapter 3 describes functions in more detail.
#### The input() Function
The [input()](https://docs.python.org/3/library/functions.html#input) function waits for the user to type some text on the keyboard and press ENTER:
```
my_name = input('>')
```
This function call evaluates to a string identical to the userâs text, and the rest of the code assigns the my\_name variable to this string value. The '\>' string passed to the function causes the \> prompt to appear, which serves as an indicator to the user that they are expected to enter something. Your programs donât have to pass a string to the input() function; if you call input(), the program will wait for the userâs text without displaying any prompt.
You can think of the input() function call as an expression that evaluates to whatever string the user typed. If the user entered 'Al', the assignment statement would effectively be my\_name \= 'Al'.
If you call input() and see an error message, like NameError: name 'Al' is not defined, the problem is that youâre running the code with Python 2 instead of Python 3.
#### The Greeting Message
The following call to print() contains the expression 'It is good to meet you, ' \+ my\_name between the parentheses:
```
print('It is good to meet you, ' + my_name)
```
Remember that expressions can always evaluate to a single value. If 'Al' is the value stored in my\_name, then this expression evaluates to 'It is good to meet you, Al'. This single string value is then passed to print(), which prints it on the screen.
#### The len() Function
You can pass the [len()](https://docs.python.org/3/library/functions.html#len) function a string value (or a variable containing a string), and the function evaluates to the integer value of the number of characters in that string:
```
print('The length of your name is:')
print(len(my_name))
```
Enter the following into the interactive shell to try this:
```
>>> len('hello')
5
>>> len('My very energetic monster just scarfed nachos.')
46
>>> len('')
0
```
Just like in those examples, len(my\_name) evaluates to an integer. We say that the len() function call *returns* or *outputs* this integer value, and the value is the function callâs *return value*. It is then passed to print() to be displayed on the screen. The print() function allows you to pass it either integer values or string values, but notice the error that shows up when you enter the following into the interactive shell:
```
>>> print('I am ' + 29 + ' years old.')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
print('I am ' + 29 + ' years old.')
TypeError: can only concatenate str (not "int") to str
```
The print() function isnât causing that error; rather, itâs the expression you tried to pass to print(). Youâll get the same error message if you type the expression into the interactive shell on its own:
```
>>> 'I am ' + 29 + ' years old.'
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
'I am ' + 29 + ' years old.'
TypeError: can only concatenate str (not "int") to str
```
Python gives an error because the \+ operator can be used only to add two numbers together or to concatenate two strings. You canât add an integer to a string because this is not allowed in Python. You can fix this by using a string version of the integer instead, as explained in the next section.
#### The str(), int(), and float() Functions
If you want to concatenate an integer such as 29 with a string to pass to print(), youâll need to get the value '29', which is the string form of 29. The str() function can be passed an integer value and will return a string value version of the integer, as follows:
```
>>> str(29)
'29'
>>> print('I am ' + str(29) + ' years old.')
I am 29 years old.
```
Because str(29) evaluates to '29', the expression 'I am ' \+ str(29) \+ ' years old.' evaluates to 'I am ' \+ '29' \+ ' years old.', which in turn evaluates to 'I am 29 years old.' This is the string value that is passed to the print() function.
The str(), int(), and float() functions will evaluate to the string, integer, and floating-point forms of the value you pass, respectively. Try converting some values in the interactive shell with these functions, and watch what happens:
```
>>> str(0)
'0'
>>> str(-3.14)
'-3.14'
>>> int('42')
42
>>> int('-99')
-99
>>> int(1.25)
1
>>> int(1.99)
1
>>> float('3.14')
3.14
>>> float(10)
10.0
```
The previous examples call the str(), int(), and float() functions and pass them values of the other data types to obtain a string, integer, or floating-point form of those values.
The str() function is handy when you have an integer or float that you want to concatenate to a string. The int() function is also helpful if you have a number as a string value that you want to use in some mathematics. For example, the input() function always returns a string, even if the user enters a number. Enter spam \= input('\>') into the interactive shell, then enter 101 when it waits for your text:
```
>>> spam = input('>')
>101
>>> spam
'101'
```
The value stored inside spam isnât the integer 101 but the string '101'. If you want to do math using the value in spam, use the int() function to get its integer form and then store this as the variableâs new value. If spam is the string '101', then the expression int(spam) will evaluate to the integer value 101, and the assignment statement spam \= int(spam) will be equivalent to spam \= 101:
```
>>> spam = int(spam)
>>> spam
101
```
Now you should be able to treat the spam variable as an integer instead of a string:
```
>>> spam * 10 / 5
202.0
```
Note that if you pass a value to int() that it cannot evaluate as an integer, Python will display an error message:
```
>>> int('99.99')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
int('99.99')
ValueError: invalid literal for int() with base 10: '99.99'
>>> int('twelve')
Traceback (most recent call last):
File "<python-input-0>", line 1, in <module>
int('twelve')
ValueError: invalid literal for int() with base 10: 'twelve'
```
The int() function is also useful if you need to round a floating-point number down:
```
>>> int(7.7)
7
>>> int(7.7) + 1
8
```
You used the int() and str() functions in the last three lines of your program to get a value of the appropriate data type for the code:
```
print('What is your age?') # Ask for their age.
my_age = input('>')
print('You will be ' + str(int(my_age) + 1) + ' in a year.')
```
The my\_age variable contains the value returned from input(). Because the input() function always returns a string (even if the user entered a number), you can use the int(my\_age) code to return an integer value of the string in my\_age. This integer value is then added to 1 in the expression int(my\_age) \+ 1.
The result of this addition is passed to the str() function: str(int(my \_age) \+ 1\). The string value returned is then concatenated with the strings 'You will be ' and ' in a year.' to evaluate to one large string value. This large string is finally passed to print() to be displayed on the screen.
Letâs say the user enters the string '4' for my\_age. The evaluation steps would look something like the following:
Description
The string '4' is converted to an integer, so you can add 1 to it. The result is 5. The str() function converts the result back to a string, so you can concatenate it with the second string, 'in a year.', to create the final message.
#### The type() Function
Integer, floating-point, and string arenât the only data types in Python. As you continue to learn about programming, you may come across values of other data types. You can always pass these to the [type()](https://docs.python.org/3/library/functions.html#type) function to determine what type they are. For example, enter the following into the interactive shell:
```
>>> type(42)
<class 'int'>
>>> type(42.0)
<class 'float'>
>>> type('forty two')
<class 'str'>
>>> name = 'Zophie'
>>> type(name) # The name variable has a value of the string type.
<class 'str'>
>>> type(len(name)) # The len() function returns integer values.
<class 'int'>
```
Not only can you pass any value to type(), but (as with any function call) you can also pass it any variable or expression to determine the data type of the value that it evaluates to. The type() function itself returns values, but the angle brackets mean they are not syntactically valid Python code; you cannot run code like spam \= \<class 'str'\>.
#### The round() and abs() Functions
Letâs learn about two more Python functions that, like the len() function, take an argument and return a value. The [round()](https://docs.python.org/3/library/functions.html#round) function accepts a float value and returns the nearest integer. Enter the following into the interactive shell:
```
>>> round(3.14)
3
>>> round(7.7)
8
>>> round(-2.2)
-2
```
The round() function also accepts an optional second argument specifying how many decimal places it should round. Enter the following into the interactive shell:
```
>>> round(3.14, 1)
3.1
>>> round(7.7777, 3)
7.778
```
The behavior for rounding half numbers is a bit odd. The function call round(3.5) rounds up to 4, while round(2.5) rounds down to 2. For halfway numbers that end with .5, the number is rounded to the nearest even integer. This is called *bankerâs rounding*.
The [abs()](https://docs.python.org/3/library/functions.html#abs) function returns the absolute value of the number argument. In mathematics, this is defined as the distance from 0, but I find it easier to think of it as the positive form of the number. Enter the following into the interactive shell:
```
>>> abs(25)
25
>>> abs(-25)
25
>>> abs(-3.14)
3.14
>>> abs(0)
0
```
Python comes with several different functions that youâll learn about in this book. This section demonstrates how you can experiment with them in the interactive shell to see how they behave with different inputs. This is a common technique for practicing the new code that you learn.
### How Computers Store Data with Binary Numbers
Thatâs enough Python code for now. At this point, you might think that programming seems almost magical. How does the computer know to transform 2 \+ 2 into 4? The answer is too complicated for this book, but I can explain part of whatâs going on behind the scenes by discussing what binary numbers (numbers that have only the digits 1 and 0) have to do with computing.
Hacking in movies often involves streams of 1s and 0s flowing across the screen. This looks mysterious and impressive, but what do these 1s and 0s actually mean? The answer is that binary is the simplest number system, and it can be implemented with inexpensive components for computer hardware. Binary, also called the *base-2 number system*, can represent all of the same numbers that our more familiar base-10 *decimal number system* can. Decimal has 10 digits, 0 through 9. Table 1-4 shows the first 27 integers in decimal and binary.
| Decimal | Binary | Decimal | Binary | Decimal | Binary |
|---|---|---|---|---|---|
| 0 | 0 | 9 | 1001 | 18 | 10010 |
| 1 | 1 | 10 | 1010 | 19 | 10011 |
| 2 | 10 | 11 | 1011 | 20 | 10100 |
| 3 | 11 | 12 | 1100 | 21 | 10101 |
| 4 | 100 | 13 | 1101 | 22 | 10110 |
| 5 | 101 | 14 | 1110 | 23 | 10111 |
| 6 | 110 | 15 | 1111 | 24 | 11000 |
| 7 | 111 | 16 | 10000 | 25 | 11001 |
| 8 | 1000 | 17 | 10001 | 26 | 11010 |
Think of these number systems as a mechanical odometer, like in Figure 1-3. When you reach the last digit, they each reset to 0 while incrementing the value of the next digit over. In decimal, the last digit is 9, and in binary, the last digit is 1. Thatâs why the decimal number after 9 is 10 and the decimal number after 999 is 1000. Similarly, the binary number after 1 is 10 and the binary number after 111 is 1000. However, *10* in binary doesnât represent the same quantity as *ten* in decimal; rather, it represents *two*. And *1000* in binary doesnât mean *one thousand* in decimal, but rather *eight*. You can view an interactive binary and decimal odometer at *[https://inventwithpython.com/odometer](https://inventwithpython.com/odometer)*.
Figure 1-3: A mechanical odometer in decimal (left) and in binary (right)
Representing binary numbers with computer hardware is simpler than representing decimal numbers because there are only two states to represent. For example, Blu-ray discs and DVDs have smooth *lands* and indented *pits* etched on their surface that will or wonât reflect the disc playerâs laser, respectively. Circuits can have electric current flowing through them or no electric current. These various hardware standards all have ways of representing two different states. On the other hand, itâd be expensive to create high-quality electronic components that are sensitive enough to detect the difference between 10 different voltage levels with reliable accuracy. Itâs more economical to use simple components, and two binary states are as simple as you can get.
These binary digits are called *bits* for short. A single bit can represent two numbers, and 8 bits (or 1 *byte*) can represent 28, or 256, numbers, ranging from 0 to 255 in decimal or 0 to 11111111 in binary. This is similar to how a single decimal digit can represent 10 numbers (0 to 9), while an eight-digit decimal number can represent 108 or 100,000,000 numbers (0 to 99,999,999). Files on your computer are measured in how many bytes they take up:
- A kilobyte (KB) is 210 or 1,024 bytes.
- A megabyte (MB) is 220 or 1,048,576 bytes (or 1,024KB).
- A gigabyte (GB) is 230 or 1,073,741,824 bytes (or 1,024MB).
- A terabyte (TB) is 240 or 1,099,511,627,776 bytes (or 1,024GB).
The text of Shakespeareâs *Romeo and Juliet* is about 135KB. A high-resolution photo is about 2MB to 5MB. A movie can be anywhere from 1GB to 50GB, depending on picture quality and movie length. However, hard drive and flash memory manufacturers blatantly lie about what these terms mean. For example, by calling a TB 1,000,000,000,000 bytes instead of 1,099,511,627,776 bytes, they can advertise a 9.09TB hard drive as 10TB.
The 1s and 0s of binary can represent not only any integer but also any form of data. Instead of 0 to 255, a byte can represent the numbers â128 to 127 using a system called *twoâs complement*. Fractional floating-point numbers can be represented in binary using a system called *IEEE-754*.
Text can be stored on computers as binary numbers by assigning each letter, punctuation mark, or symbol a unique number. A system for representing text as numbers is called an *encoding*. The most popular encoding for text is UTF-8. In UTF-8, a capital letter *A* is represented by the decimal number 65 (or 01000001 as an 8-bit binary number), a *?* (question mark) is represented by the number 63, and the numeral character *7* is represented by the number 55. The string 'Hello' is stored as the numbers 72, 101, 108, 108, and 111. When stored in a computer, âHelloâ appears as a stream of bits: 0100100001100101011011000110110001101111.
Wow! Just like in those hacker movies\!
Engineers need to invent a way to encode each form of data as numbers. Photos and images can be broken up into a two-dimensional grid of colored squares called *pixels*. Each pixel can use three bytes to represent how much red, green, and blue color it contains. (Chapter 21 covers image data in more detail.) But for a short example, the numbers 255, 0, and 255 could represent a pixel with the maximum amount of red and blue but zero green, resulting in a purple pixel.
Sound is made up of waves of compressed air that reach our ears, which our brains interpret as audio sensation. We can graph the intensity and frequency of these waves over time. The numbers on this graph can then be converted to binary numbers and stored on a computer, which later control speakers to reproduce the sound. This is a simplification of how computer audio works, but describes how numbers can represent Beethovenâs Symphony No. 5.
The data for several images combines with audio data to store videos. All forms of information can be encoded into binary numbers. There is, of course, a great deal more detail to it than this, but this is how 1s and 0s represent the wide variety of data in our information age.
### Summary
You can compute expressions with a calculator or enter string concatenations with a word processor. You can even do string replication easily by copying and pasting text. But expressions, and their component valuesâoperators, variables, and function callsâare the basic building blocks that make up programs. Once you know how to handle these elements, you will be able to instruct Python to operate on large amounts of data for you.
Youâll find it helpful to remember the different types of operators (\+, \-, \*, /, //, %, and \*\* for math operations, and \+ and \* for string operations) and the three data types (integers, floating-point numbers, and strings) introduced in this chapter.
I introduced a few different functions as well. The print() and input() functions handle simple text output (to the screen) and input (from the keyboard). The len() function takes a string and evaluates to an int of the number of characters in the string. The str(), int(), and float() functions will evaluate to the string, integer, or floating-point number form of the value they are passed. The round() function returns the rounded integer, and the abs() function returns the absolute value of the arguments.
In the next chapter, youâll learn how to tell Python to make intelligent decisions about what code to run, what code to skip, and what code to repeat based on the values it has. This is known as *flow control*, and it allows you to write programs that make intelligent decisions.
### Practice Questions
1\. Which of the following are operators, and which are values?
```
*
'hello'
-88.8
-
/
+
5
```
2\. Which of the following is a variable, and which is a string?
```
spam
'spam'
```
3\. Name three data types.
4\. What is an expression made up of? What do all expressions do?
5\. This chapter introduced assignment statements, like spam \= 10. What is the difference between an expression and a statement?
6\. What does the variable bacon contain after the following code runs?
```
bacon = 20
bacon + 1
```
7\. What should the following two expressions evaluate to?
```
'spam' + 'spamspam'
'spam' * 3
```
8\. Why is eggs a valid variable name while 100 is invalid?
9\. What three functions can be used to get the integer, floating-point number, or string version of a value?
10\. Why does this expression cause an error? How can you fix it?
```
'I eat ' + 99 + ' burritos.'
```
**Extra credit:** Search online for the Python documentation for the len() function. It will be on a web page titled âBuilt-in Functions.â Skim the list of other functions Python has, look up what the bin() and hex() functions do, and experiment with them in the interactive shell. |
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