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| Boilerpipe Text | Until now error messages havenât been more than mentioned, but if you have tried
out the examples you have probably seen some. There are (at least) two
distinguishable kinds of errors:
syntax errors
and
exceptions
.
8.1.
Syntax Errors
¶
Syntax errors, also known as parsing errors, are perhaps the most common kind of
complaint you get while you are still learning Python:
>>>
while
True
print
(
'Hello world'
)
File
"<stdin>"
, line
1
while
True
print
(
'Hello world'
)
^^^^^
SyntaxError
:
invalid syntax
The parser repeats the offending line and displays little arrows pointing
at the place where the error was detected. Note that this is not always the
place that needs to be fixed. In the example, the error is detected at the
function
print()
, since a colon (
':'
) is missing just before it.
The file name (
<stdin>
in our example) and line number are printed so you
know where to look in case the input came from a file.
8.2.
Exceptions
¶
Even if a statement or expression is syntactically correct, it may cause an
error when an attempt is made to execute it. Errors detected during execution
are called
exceptions
and are not unconditionally fatal: you will soon learn
how to handle them in Python programs. Most exceptions are not handled by
programs, however, and result in error messages as shown here:
>>>
10
*
(
1
/
0
)
Traceback (most recent call last):
File
"<stdin>"
, line
1
, in
<module>
10
*
(
1
/
0
)
~^~
ZeroDivisionError
:
division by zero
>>>
4
+
spam
*
3
Traceback (most recent call last):
File
"<stdin>"
, line
1
, in
<module>
4
+
spam
*
3
^^^^
NameError
:
name 'spam' is not defined
>>>
'2'
+
2
Traceback (most recent call last):
File
"<stdin>"
, line
1
, in
<module>
'2'
+
2
~~~~^~~
TypeError
:
can only concatenate str (not "int") to str
The last line of the error message indicates what happened. Exceptions come in
different types, and the type is printed as part of the message: the types in
the example are
ZeroDivisionError
,
NameError
and
TypeError
.
The string printed as the exception type is the name of the built-in exception
that occurred. This is true for all built-in exceptions, but need not be true
for user-defined exceptions (although it is a useful convention). Standard
exception names are built-in identifiers (not reserved keywords).
The rest of the line provides detail based on the type of exception and what
caused it.
The preceding part of the error message shows the context where the exception
occurred, in the form of a stack traceback. In general it contains a stack
traceback listing source lines; however, it will not display lines read from
standard input.
Built-in Exceptions
lists the built-in exceptions and their meanings.
8.3.
Handling Exceptions
¶
It is possible to write programs that handle selected exceptions. Look at the
following example, which asks the user for input until a valid integer has been
entered, but allows the user to interrupt the program (using
Control
-
C
or
whatever the operating system supports); note that a user-generated interruption
is signalled by raising the
KeyboardInterrupt
exception.
>>>
while
True
:
...
try
:
...
x
=
int
(
input
(
"Please enter a number: "
))
...
break
...
except
ValueError
:
...
print
(
"Oops! That was no valid number. Try again..."
)
...
The
try
statement works as follows.
First, the
try clause
(the statement(s) between the
try
and
except
keywords) is executed.
If no exception occurs, the
except clause
is skipped and execution of the
try
statement is finished.
If an exception occurs during execution of the
try
clause, the rest of the
clause is skipped. Then, if its type matches the exception named after the
except
keyword, the
except clause
is executed, and then execution
continues after the try/except block.
If an exception occurs which does not match the exception named in the
except
clause
, it is passed on to outer
try
statements; if no handler is
found, it is an
unhandled exception
and execution stops with an error message.
A
try
statement may have more than one
except clause
, to specify
handlers for different exceptions. At most one handler will be executed.
Handlers only handle exceptions that occur in the corresponding
try clause
,
not in other handlers of the same
try
statement. An
except clause
may name multiple exceptions, for example:
...
except
RuntimeError
,
TypeError
,
NameError
:
...
pass
A class in an
except
clause matches exceptions which are instances of the
class itself or one of its derived classes (but not the other way around â an
except clause
listing a derived class does not match instances of its base classes).
For example, the following code will print B, C, D in that order:
class
B
(
Exception
):
pass
class
C
(
B
):
pass
class
D
(
C
):
pass
for
cls
in
[
B
,
C
,
D
]:
try
:
raise
cls
()
except
D
:
print
(
"D"
)
except
C
:
print
(
"C"
)
except
B
:
print
(
"B"
)
Note that if the
except clauses
were reversed (with
except
B
first), it
would have printed B, B, B â the first matching
except clause
is triggered.
When an exception occurs, it may have associated values, also known as the
exceptionâs
arguments
. The presence and types of the arguments depend on the
exception type.
The
except clause
may specify a variable after the exception name. The
variable is bound to the exception instance which typically has an
args
attribute that stores the arguments. For convenience, builtin exception
types define
__str__()
to print all the arguments without explicitly
accessing
.args
.
>>>
try
:
...
raise
Exception
(
'spam'
,
'eggs'
)
...
except
Exception
as
inst
:
...
print
(
type
(
inst
))
# the exception type
...
print
(
inst
.
args
)
# arguments stored in .args
...
print
(
inst
)
# __str__ allows args to be printed directly,
...
# but may be overridden in exception subclasses
...
x
,
y
=
inst
.
args
# unpack args
...
print
(
'x ='
,
x
)
...
print
(
'y ='
,
y
)
...
<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs
The exceptionâs
__str__()
output is printed as the last part (âdetailâ)
of the message for unhandled exceptions.
BaseException
is the common base class of all exceptions. One of its
subclasses,
Exception
, is the base class of all the non-fatal exceptions.
Exceptions which are not subclasses of
Exception
are not typically
handled, because they are used to indicate that the program should terminate.
They include
SystemExit
which is raised by
sys.exit()
and
KeyboardInterrupt
which is raised when a user wishes to interrupt
the program.
Exception
can be used as a wildcard that catches (almost) everything.
However, it is good practice to be as specific as possible with the types
of exceptions that we intend to handle, and to allow any unexpected
exceptions to propagate on.
The most common pattern for handling
Exception
is to print or log
the exception and then re-raise it (allowing a caller to handle the
exception as well):
import
sys
try
:
f
=
open
(
'myfile.txt'
)
s
=
f
.
readline
()
i
=
int
(
s
.
strip
())
except
OSError
as
err
:
print
(
"OS error:"
,
err
)
except
ValueError
:
print
(
"Could not convert data to an integer."
)
except
Exception
as
err
:
print
(
f
"Unexpected
{
err
=}
,
{
type
(
err
)
=}
"
)
raise
The
try
âŠ
except
statement has an optional
else
clause
, which, when present, must follow all
except clauses
. It is useful
for code that must be executed if the
try clause
does not raise an exception.
For example:
for
arg
in
sys
.
argv
[
1
:]:
try
:
f
=
open
(
arg
,
'r'
)
except
OSError
:
print
(
'cannot open'
,
arg
)
else
:
print
(
arg
,
'has'
,
len
(
f
.
readlines
()),
'lines'
)
f
.
close
()
The use of the
else
clause is better than adding additional code to
the
try
clause because it avoids accidentally catching an exception
that wasnât raised by the code being protected by the
try
âŠ
except
statement.
Exception handlers do not handle only exceptions that occur immediately in the
try clause
, but also those that occur inside functions that are called (even
indirectly) in the
try clause
. For example:
>>>
def
this_fails
():
...
x
=
1
/
0
...
>>>
try
:
...
this_fails
()
...
except
ZeroDivisionError
as
err
:
...
print
(
'Handling run-time error:'
,
err
)
...
Handling run-time error: division by zero
8.4.
Raising Exceptions
¶
The
raise
statement allows the programmer to force a specified
exception to occur. For example:
>>>
raise
NameError
(
'HiThere'
)
Traceback (most recent call last):
File
"<stdin>"
, line
1
, in
<module>
raise
NameError
(
'HiThere'
)
NameError
:
HiThere
The sole argument to
raise
indicates the exception to be raised.
This must be either an exception instance or an exception class (a class that
derives from
BaseException
, such as
Exception
or one of its
subclasses). If an exception class is passed, it will be implicitly
instantiated by calling its constructor with no arguments:
raise
ValueError
# shorthand for 'raise ValueError()'
If you need to determine whether an exception was raised but donât intend to
handle it, a simpler form of the
raise
statement allows you to
re-raise the exception:
>>>
try
:
...
raise
NameError
(
'HiThere'
)
...
except
NameError
:
...
print
(
'An exception flew by!'
)
...
raise
...
An exception flew by!
Traceback (most recent call last):
File
"<stdin>"
, line
2
, in
<module>
raise
NameError
(
'HiThere'
)
NameError
:
HiThere
8.5.
Exception Chaining
¶
If an unhandled exception occurs inside an
except
section, it will
have the exception being handled attached to it and included in the error
message:
>>>
try
:
...
open
(
"database.sqlite"
)
...
except
OSError
:
...
raise
RuntimeError
(
"unable to handle error"
)
...
Traceback (most recent call last):
File
"<stdin>"
, line
2
, in
<module>
open
(
"database.sqlite"
)
~~~~^^^^^^^^^^^^^^^^^^^
FileNotFoundError
:
[Errno 2] No such file or directory: 'database.sqlite'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File
"<stdin>"
, line
4
, in
<module>
raise
RuntimeError
(
"unable to handle error"
)
RuntimeError
:
unable to handle error
To indicate that an exception is a direct consequence of another, the
raise
statement allows an optional
from
clause:
# exc must be exception instance or None.
raise
RuntimeError
from
exc
This can be useful when you are transforming exceptions. For example:
>>>
def
func
():
...
raise
ConnectionError
...
>>>
try
:
...
func
()
...
except
ConnectionError
as
exc
:
...
raise
RuntimeError
(
'Failed to open database'
)
from
exc
...
Traceback (most recent call last):
File
"<stdin>"
, line
2
, in
<module>
func
()
~~~~^^
File
"<stdin>"
, line
2
, in
func
ConnectionError
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File
"<stdin>"
, line
4
, in
<module>
raise
RuntimeError
(
'Failed to open database'
)
from
exc
RuntimeError
:
Failed to open database
It also allows disabling automatic exception chaining using the
from
None
idiom:
>>>
try
:
...
open
(
'database.sqlite'
)
...
except
OSError
:
...
raise
RuntimeError
from
None
...
Traceback (most recent call last):
File
"<stdin>"
, line
4
, in
<module>
raise
RuntimeError
from
None
RuntimeError
For more information about chaining mechanics, see
Built-in Exceptions
.
8.6.
User-defined Exceptions
¶
Programs may name their own exceptions by creating a new exception class (see
Classes
for more about Python classes). Exceptions should typically
be derived from the
Exception
class, either directly or indirectly.
Exception classes can be defined which do anything any other class can do, but
are usually kept simple, often only offering a number of attributes that allow
information about the error to be extracted by handlers for the exception.
Most exceptions are defined with names that end in âErrorâ, similar to the
naming of the standard exceptions.
Many standard modules define their own exceptions to report errors that may
occur in functions they define.
8.7.
Defining Clean-up Actions
¶
The
try
statement has another optional clause which is intended to
define clean-up actions that must be executed under all circumstances. For
example:
>>>
try
:
...
raise
KeyboardInterrupt
...
finally
:
...
print
(
'Goodbye, world!'
)
...
Goodbye, world!
Traceback (most recent call last):
File
"<stdin>"
, line
2
, in
<module>
raise
KeyboardInterrupt
KeyboardInterrupt
If a
finally
clause is present, the
finally
clause will execute as the last task before the
try
statement completes. The
finally
clause runs whether or
not the
try
statement produces an exception. The following
points discuss more complex cases when an exception occurs:
If an exception occurs during execution of the
try
clause, the exception may be handled by an
except
clause. If the exception is not handled by an
except
clause, the exception is re-raised after the
finally
clause has been executed.
An exception could occur during execution of an
except
or
else
clause. Again, the exception is re-raised after
the
finally
clause has been executed.
If the
finally
clause executes a
break
,
continue
or
return
statement, exceptions are not
re-raised. This can be confusing and is therefore discouraged. From
version 3.14 the compiler emits a
SyntaxWarning
for it
(see
PEP 765
).
If the
try
statement reaches a
break
,
continue
or
return
statement, the
finally
clause will execute just prior to the
break
,
continue
or
return
statementâs execution.
If a
finally
clause includes a
return
statement, the returned value will be the one from the
finally
clauseâs
return
statement, not the
value from the
try
clauseâs
return
statement. This can be confusing and is therefore discouraged. From
version 3.14 the compiler emits a
SyntaxWarning
for it
(see
PEP 765
).
For example:
>>>
def
bool_return
():
...
try
:
...
return
True
...
finally
:
...
return
False
...
>>>
bool_return
()
False
A more complicated example:
>>>
def
divide
(
x
,
y
):
...
try
:
...
result
=
x
/
y
...
except
ZeroDivisionError
:
...
print
(
"division by zero!"
)
...
else
:
...
print
(
"result is"
,
result
)
...
finally
:
...
print
(
"executing finally clause"
)
...
>>>
divide
(
2
,
1
)
result is 2.0
executing finally clause
>>>
divide
(
2
,
0
)
division by zero!
executing finally clause
>>>
divide
(
"2"
,
"1"
)
executing finally clause
Traceback (most recent call last):
File
"<stdin>"
, line
1
, in
<module>
divide
(
"2"
,
"1"
)
~~~~~~^^^^^^^^^^
File
"<stdin>"
, line
3
, in
divide
result
=
x
/
y
~~^~~
TypeError
:
unsupported operand type(s) for /: 'str' and 'str'
As you can see, the
finally
clause is executed in any event. The
TypeError
raised by dividing two strings is not handled by the
except
clause and therefore re-raised after the
finally
clause has been executed.
In real world applications, the
finally
clause is useful for
releasing external resources (such as files or network connections), regardless
of whether the use of the resource was successful.
8.8.
Predefined Clean-up Actions
¶
Some objects define standard clean-up actions to be undertaken when the object
is no longer needed, regardless of whether or not the operation using the object
succeeded or failed. Look at the following example, which tries to open a file
and print its contents to the screen.
for
line
in
open
(
"myfile.txt"
):
print
(
line
,
end
=
""
)
The problem with this code is that it leaves the file open for an indeterminate
amount of time after this part of the code has finished executing.
This is not an issue in simple scripts, but can be a problem for larger
applications. The
with
statement allows objects like files to be
used in a way that ensures they are always cleaned up promptly and correctly.
with
open
(
"myfile.txt"
)
as
f
:
for
line
in
f
:
print
(
line
,
end
=
""
)
After the statement is executed, the file
f
is always closed, even if a
problem was encountered while processing the lines. Objects which, like files,
provide predefined clean-up actions will indicate this in their documentation.
8.9.
Raising and Handling Multiple Unrelated Exceptions
¶
There are situations where it is necessary to report several exceptions that
have occurred. This is often the case in concurrency frameworks, when several
tasks may have failed in parallel, but there are also other use cases where
it is desirable to continue execution and collect multiple errors rather than
raise the first exception.
The builtin
ExceptionGroup
wraps a list of exception instances so
that they can be raised together. It is an exception itself, so it can be
caught like any other exception.
>>>
def
f
():
...
excs
=
[
OSError
(
'error 1'
),
SystemError
(
'error 2'
)]
...
raise
ExceptionGroup
(
'there were problems'
,
excs
)
...
>>>
f
()
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 1, in <module>
| f()
| ~^^
| File "<stdin>", line 3, in f
| raise ExceptionGroup('there were problems', excs)
| ExceptionGroup: there were problems (2 sub-exceptions)
+-+---------------- 1 ----------------
| OSError: error 1
+---------------- 2 ----------------
| SystemError: error 2
+------------------------------------
>>>
try
:
...
f
()
...
except
Exception
as
e
:
...
print
(
f
'caught
{
type
(
e
)
}
:
{
e
}
'
)
...
caught <class 'ExceptionGroup'>: there were problems (2 sub-exceptions)
>>>
By using
except*
instead of
except
, we can selectively
handle only the exceptions in the group that match a certain
type. In the following example, which shows a nested exception
group, each
except*
clause extracts from the group exceptions
of a certain type while letting all other exceptions propagate to
other clauses and eventually to be reraised.
>>>
def
f
():
...
raise
ExceptionGroup
(
...
"group1"
,
...
[
...
OSError
(
1
),
...
SystemError
(
2
),
...
ExceptionGroup
(
...
"group2"
,
...
[
...
OSError
(
3
),
...
RecursionError
(
4
)
...
]
...
)
...
]
...
)
...
>>>
try
:
...
f
()
...
except
*
OSError
as
e
:
...
print
(
"There were OSErrors"
)
...
except
*
SystemError
as
e
:
...
print
(
"There were SystemErrors"
)
...
There were OSErrors
There were SystemErrors
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 2, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise ExceptionGroup(
| ...<12 lines>...
| )
| ExceptionGroup: group1 (1 sub-exception)
+-+---------------- 1 ----------------
| ExceptionGroup: group2 (1 sub-exception)
+-+---------------- 1 ----------------
| RecursionError: 4
+------------------------------------
>>>
Note that the exceptions nested in an exception group must be instances,
not types. This is because in practice the exceptions would typically
be ones that have already been raised and caught by the program, along
the following pattern:
>>>
excs
=
[]
...
for
test
in
tests
:
...
try
:
...
test
.
run
()
...
except
Exception
as
e
:
...
excs
.
append
(
e
)
...
>>>
if
excs
:
...
raise
ExceptionGroup
(
"Test Failures"
,
excs
)
...
8.10.
Enriching Exceptions with Notes
¶
When an exception is created in order to be raised, it is usually initialized
with information that describes the error that has occurred. There are cases
where it is useful to add information after the exception was caught. For this
purpose, exceptions have a method
add_note(note)
that accepts a string and
adds it to the exceptionâs notes list. The standard traceback rendering
includes all notes, in the order they were added, after the exception.
>>>
try
:
...
raise
TypeError
(
'bad type'
)
...
except
Exception
as
e
:
...
e
.
add_note
(
'Add some information'
)
...
e
.
add_note
(
'Add some more information'
)
...
raise
...
Traceback (most recent call last):
File
"<stdin>"
, line
2
, in
<module>
raise
TypeError
(
'bad type'
)
TypeError
:
bad type
Add some information
Add some more information
>>>
For example, when collecting exceptions into an exception group, we may want
to add context information for the individual errors. In the following each
exception in the group has a note indicating when this error has occurred.
>>>
def
f
():
...
raise
OSError
(
'operation failed'
)
...
>>>
excs
=
[]
>>>
for
i
in
range
(
3
):
...
try
:
...
f
()
...
except
Exception
as
e
:
...
e
.
add_note
(
f
'Happened in Iteration
{
i
+
1
}
'
)
...
excs
.
append
(
e
)
...
>>>
raise
ExceptionGroup
(
'We have some problems'
,
excs
)
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 1, in <module>
| raise ExceptionGroup('We have some problems', excs)
| ExceptionGroup: We have some problems (3 sub-exceptions)
+-+---------------- 1 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 1
+---------------- 2 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 2
+---------------- 3 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 3
+------------------------------------
>>> |
| Markdown | [](https://www.python.org/)
Theme
### [Table of Contents](https://docs.python.org/3/contents.html)
- [8\. Errors and Exceptions](https://docs.python.org/3/tutorial/errors.html)
- [8\.1. Syntax Errors](https://docs.python.org/3/tutorial/errors.html#syntax-errors)
- [8\.2. Exceptions](https://docs.python.org/3/tutorial/errors.html#exceptions)
- [8\.3. Handling Exceptions](https://docs.python.org/3/tutorial/errors.html#handling-exceptions)
- [8\.4. Raising Exceptions](https://docs.python.org/3/tutorial/errors.html#raising-exceptions)
- [8\.5. Exception Chaining](https://docs.python.org/3/tutorial/errors.html#exception-chaining)
- [8\.6. User-defined Exceptions](https://docs.python.org/3/tutorial/errors.html#user-defined-exceptions)
- [8\.7. Defining Clean-up Actions](https://docs.python.org/3/tutorial/errors.html#defining-clean-up-actions)
- [8\.8. Predefined Clean-up Actions](https://docs.python.org/3/tutorial/errors.html#predefined-clean-up-actions)
- [8\.9. Raising and Handling Multiple Unrelated Exceptions](https://docs.python.org/3/tutorial/errors.html#raising-and-handling-multiple-unrelated-exceptions)
- [8\.10. Enriching Exceptions with Notes](https://docs.python.org/3/tutorial/errors.html#enriching-exceptions-with-notes)
#### Previous topic
[7\. Input and Output](https://docs.python.org/3/tutorial/inputoutput.html "previous chapter")
#### Next topic
[9\. Classes](https://docs.python.org/3/tutorial/classes.html "next chapter")
### This page
- [Report a bug](https://docs.python.org/3/bugs.html)
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# 8\. Errors and Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#errors-and-exceptions "Link to this heading")
Until now error messages havenât been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: *syntax errors* and *exceptions*.
## 8\.1. Syntax Errors[¶](https://docs.python.org/3/tutorial/errors.html#syntax-errors "Link to this heading")
Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python:
Copy
```
>>> while True print('Hello world')
File "<stdin>", line 1
while True print('Hello world')
^^^^^
SyntaxError: invalid syntax
```
The parser repeats the offending line and displays little arrows pointing at the place where the error was detected. Note that this is not always the place that needs to be fixed. In the example, the error is detected at the function [`print()`](https://docs.python.org/3/library/functions.html#print "print"), since a colon (`':'`) is missing just before it.
The file name (`<stdin>` in our example) and line number are printed so you know where to look in case the input came from a file.
## 8\.2. Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#exceptions "Link to this heading")
Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called *exceptions* and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here:
Copy
```
>>> 10 * (1/0)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
10 * (1/0)
~^~
ZeroDivisionError: division by zero
>>> 4 + spam*3
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
4 + spam*3
^^^^
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
'2' + 2
~~~~^~~
TypeError: can only concatenate str (not "int") to str
```
The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are [`ZeroDivisionError`](https://docs.python.org/3/library/exceptions.html#ZeroDivisionError "ZeroDivisionError"), [`NameError`](https://docs.python.org/3/library/exceptions.html#NameError "NameError") and [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError"). The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords).
The rest of the line provides detail based on the type of exception and what caused it.
The preceding part of the error message shows the context where the exception occurred, in the form of a stack traceback. In general it contains a stack traceback listing source lines; however, it will not display lines read from standard input.
[Built-in Exceptions](https://docs.python.org/3/library/exceptions.html#bltin-exceptions) lists the built-in exceptions and their meanings.
## 8\.3. Handling Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#handling-exceptions "Link to this heading")
It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using `Control`\-`C` or whatever the operating system supports); note that a user-generated interruption is signalled by raising the [`KeyboardInterrupt`](https://docs.python.org/3/library/exceptions.html#KeyboardInterrupt "KeyboardInterrupt") exception.
Copy
```
>>> while True:
... try:
... x = int(input("Please enter a number: "))
... break
... except ValueError:
... print("Oops! That was no valid number. Try again...")
...
```
The [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement works as follows.
- First, the *try clause* (the statement(s) between the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) and [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) keywords) is executed.
- If no exception occurs, the *except clause* is skipped and execution of the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement is finished.
- If an exception occurs during execution of the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) clause, the rest of the clause is skipped. Then, if its type matches the exception named after the [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) keyword, the *except clause* is executed, and then execution continues after the try/except block.
- If an exception occurs which does not match the exception named in the *except clause*, it is passed on to outer [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statements; if no handler is found, it is an *unhandled exception* and execution stops with an error message.
A [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement may have more than one *except clause*, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding *try clause*, not in other handlers of the same `try` statement. An *except clause* may name multiple exceptions, for example:
Copy
```
... except RuntimeError, TypeError, NameError:
... pass
```
A class in an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) clause matches exceptions which are instances of the class itself or one of its derived classes (but not the other way around â an *except clause* listing a derived class does not match instances of its base classes). For example, the following code will print B, C, D in that order:
Copy
```
class B(Exception):
pass
class C(B):
pass
class D(C):
pass
for cls in [B, C, D]:
try:
raise cls()
except D:
print("D")
except C:
print("C")
except B:
print("B")
```
Note that if the *except clauses* were reversed (with `except B` first), it would have printed B, B, B â the first matching *except clause* is triggered.
When an exception occurs, it may have associated values, also known as the exceptionâs *arguments*. The presence and types of the arguments depend on the exception type.
The *except clause* may specify a variable after the exception name. The variable is bound to the exception instance which typically has an `args` attribute that stores the arguments. For convenience, builtin exception types define [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__") to print all the arguments without explicitly accessing `.args`.
Copy
```
>>> try:
... raise Exception('spam', 'eggs')
... except Exception as inst:
... print(type(inst)) # the exception type
... print(inst.args) # arguments stored in .args
... print(inst) # __str__ allows args to be printed directly,
... # but may be overridden in exception subclasses
... x, y = inst.args # unpack args
... print('x =', x)
... print('y =', y)
...
<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs
```
The exceptionâs [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__") output is printed as the last part (âdetailâ) of the message for unhandled exceptions.
[`BaseException`](https://docs.python.org/3/library/exceptions.html#BaseException "BaseException") is the common base class of all exceptions. One of its subclasses, [`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception"), is the base class of all the non-fatal exceptions. Exceptions which are not subclasses of `Exception` are not typically handled, because they are used to indicate that the program should terminate. They include [`SystemExit`](https://docs.python.org/3/library/exceptions.html#SystemExit "SystemExit") which is raised by [`sys.exit()`](https://docs.python.org/3/library/sys.html#sys.exit "sys.exit") and [`KeyboardInterrupt`](https://docs.python.org/3/library/exceptions.html#KeyboardInterrupt "KeyboardInterrupt") which is raised when a user wishes to interrupt the program.
[`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception") can be used as a wildcard that catches (almost) everything. However, it is good practice to be as specific as possible with the types of exceptions that we intend to handle, and to allow any unexpected exceptions to propagate on.
The most common pattern for handling [`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception") is to print or log the exception and then re-raise it (allowing a caller to handle the exception as well):
Copy
```
import sys
try:
f = open('myfile.txt')
s = f.readline()
i = int(s.strip())
except OSError as err:
print("OS error:", err)
except ValueError:
print("Could not convert data to an integer.")
except Exception as err:
print(f"Unexpected {err=}, {type(err)=}")
raise
```
The [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) ⊠[`except`](https://docs.python.org/3/reference/compound_stmts.html#except) statement has an optional *else clause*, which, when present, must follow all *except clauses*. It is useful for code that must be executed if the *try clause* does not raise an exception. For example:
Copy
```
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except OSError:
print('cannot open', arg)
else:
print(arg, 'has', len(f.readlines()), 'lines')
f.close()
```
The use of the `else` clause is better than adding additional code to the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) clause because it avoids accidentally catching an exception that wasnât raised by the code being protected by the `try` ⊠`except` statement.
Exception handlers do not handle only exceptions that occur immediately in the *try clause*, but also those that occur inside functions that are called (even indirectly) in the *try clause*. For example:
Copy
```
>>> def this_fails():
... x = 1/0
...
>>> try:
... this_fails()
... except ZeroDivisionError as err:
... print('Handling run-time error:', err)
...
Handling run-time error: division by zero
```
## 8\.4. Raising Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#raising-exceptions "Link to this heading")
The [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) statement allows the programmer to force a specified exception to occur. For example:
Copy
```
>>> raise NameError('HiThere')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
raise NameError('HiThere')
NameError: HiThere
```
The sole argument to [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from [`BaseException`](https://docs.python.org/3/library/exceptions.html#BaseException "BaseException"), such as [`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception") or one of its subclasses). If an exception class is passed, it will be implicitly instantiated by calling its constructor with no arguments:
Copy
```
raise ValueError # shorthand for 'raise ValueError()'
```
If you need to determine whether an exception was raised but donât intend to handle it, a simpler form of the [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) statement allows you to re-raise the exception:
Copy
```
>>> try:
... raise NameError('HiThere')
... except NameError:
... print('An exception flew by!')
... raise
...
An exception flew by!
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
raise NameError('HiThere')
NameError: HiThere
```
## 8\.5. Exception Chaining[¶](https://docs.python.org/3/tutorial/errors.html#exception-chaining "Link to this heading")
If an unhandled exception occurs inside an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) section, it will have the exception being handled attached to it and included in the error message:
Copy
```
>>> try:
... open("database.sqlite")
... except OSError:
... raise RuntimeError("unable to handle error")
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
open("database.sqlite")
~~~~^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'database.sqlite'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError("unable to handle error")
RuntimeError: unable to handle error
```
To indicate that an exception is a direct consequence of another, the [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) statement allows an optional `from` clause:
Copy
```
# exc must be exception instance or None.
raise RuntimeError from exc
```
This can be useful when you are transforming exceptions. For example:
Copy
```
>>> def func():
... raise ConnectionError
...
>>> try:
... func()
... except ConnectionError as exc:
... raise RuntimeError('Failed to open database') from exc
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
func()
~~~~^^
File "<stdin>", line 2, in func
ConnectionError
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError('Failed to open database') from exc
RuntimeError: Failed to open database
```
It also allows disabling automatic exception chaining using the `from None` idiom:
Copy
```
>>> try:
... open('database.sqlite')
... except OSError:
... raise RuntimeError from None
...
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError from None
RuntimeError
```
For more information about chaining mechanics, see [Built-in Exceptions](https://docs.python.org/3/library/exceptions.html#bltin-exceptions).
## 8\.6. User-defined Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#user-defined-exceptions "Link to this heading")
Programs may name their own exceptions by creating a new exception class (see [Classes](https://docs.python.org/3/tutorial/classes.html#tut-classes) for more about Python classes). Exceptions should typically be derived from the [`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception") class, either directly or indirectly.
Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception.
Most exceptions are defined with names that end in âErrorâ, similar to the naming of the standard exceptions.
Many standard modules define their own exceptions to report errors that may occur in functions they define.
## 8\.7. Defining Clean-up Actions[¶](https://docs.python.org/3/tutorial/errors.html#defining-clean-up-actions "Link to this heading")
The [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:
Copy
```
>>> try:
... raise KeyboardInterrupt
... finally:
... print('Goodbye, world!')
...
Goodbye, world!
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
raise KeyboardInterrupt
KeyboardInterrupt
```
If a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause is present, the `finally` clause will execute as the last task before the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement completes. The `finally` clause runs whether or not the `try` statement produces an exception. The following points discuss more complex cases when an exception occurs:
- If an exception occurs during execution of the `try` clause, the exception may be handled by an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) clause. If the exception is not handled by an `except` clause, the exception is re-raised after the `finally` clause has been executed.
- An exception could occur during execution of an `except` or `else` clause. Again, the exception is re-raised after the `finally` clause has been executed.
- If the `finally` clause executes a [`break`](https://docs.python.org/3/reference/simple_stmts.html#break), [`continue`](https://docs.python.org/3/reference/simple_stmts.html#continue) or [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement, exceptions are not re-raised. This can be confusing and is therefore discouraged. From version 3.14 the compiler emits a [`SyntaxWarning`](https://docs.python.org/3/library/exceptions.html#SyntaxWarning "SyntaxWarning") for it (see [**PEP 765**](https://peps.python.org/pep-0765/)).
- If the `try` statement reaches a [`break`](https://docs.python.org/3/reference/simple_stmts.html#break), [`continue`](https://docs.python.org/3/reference/simple_stmts.html#continue) or [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement, the `finally` clause will execute just prior to the `break`, `continue` or `return` statementâs execution.
- If a `finally` clause includes a `return` statement, the returned value will be the one from the `finally` clauseâs `return` statement, not the value from the `try` clauseâs `return` statement. This can be confusing and is therefore discouraged. From version 3.14 the compiler emits a [`SyntaxWarning`](https://docs.python.org/3/library/exceptions.html#SyntaxWarning "SyntaxWarning") for it (see [**PEP 765**](https://peps.python.org/pep-0765/)).
For example:
Copy
```
>>> def bool_return():
... try:
... return True
... finally:
... return False
...
>>> bool_return()
False
```
A more complicated example:
Copy
```
>>> def divide(x, y):
... try:
... result = x / y
... except ZeroDivisionError:
... print("division by zero!")
... else:
... print("result is", result)
... finally:
... print("executing finally clause")
...
>>> divide(2, 1)
result is 2.0
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
divide("2", "1")
~~~~~~^^^^^^^^^^
File "<stdin>", line 3, in divide
result = x / y
~~^~~
TypeError: unsupported operand type(s) for /: 'str' and 'str'
```
As you can see, the [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause is executed in any event. The [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") raised by dividing two strings is not handled by the [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) clause and therefore re-raised after the `finally` clause has been executed.
In real world applications, the [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.
## 8\.8. Predefined Clean-up Actions[¶](https://docs.python.org/3/tutorial/errors.html#predefined-clean-up-actions "Link to this heading")
Some objects define standard clean-up actions to be undertaken when the object is no longer needed, regardless of whether or not the operation using the object succeeded or failed. Look at the following example, which tries to open a file and print its contents to the screen.
Copy
```
for line in open("myfile.txt"):
print(line, end="")
```
The problem with this code is that it leaves the file open for an indeterminate amount of time after this part of the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. The [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.
Copy
```
with open("myfile.txt") as f:
for line in f:
print(line, end="")
```
After the statement is executed, the file *f* is always closed, even if a problem was encountered while processing the lines. Objects which, like files, provide predefined clean-up actions will indicate this in their documentation.
## 8\.9. Raising and Handling Multiple Unrelated Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#raising-and-handling-multiple-unrelated-exceptions "Link to this heading")
There are situations where it is necessary to report several exceptions that have occurred. This is often the case in concurrency frameworks, when several tasks may have failed in parallel, but there are also other use cases where it is desirable to continue execution and collect multiple errors rather than raise the first exception.
The builtin [`ExceptionGroup`](https://docs.python.org/3/library/exceptions.html#ExceptionGroup "ExceptionGroup") wraps a list of exception instances so that they can be raised together. It is an exception itself, so it can be caught like any other exception.
Copy
```
>>> def f():
... excs = [OSError('error 1'), SystemError('error 2')]
... raise ExceptionGroup('there were problems', excs)
...
>>> f()
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 1, in <module>
| f()
| ~^^
| File "<stdin>", line 3, in f
| raise ExceptionGroup('there were problems', excs)
| ExceptionGroup: there were problems (2 sub-exceptions)
+-+---------------- 1 ----------------
| OSError: error 1
+---------------- 2 ----------------
| SystemError: error 2
+------------------------------------
>>> try:
... f()
... except Exception as e:
... print(f'caught {type(e)}: {e}')
...
caught <class 'ExceptionGroup'>: there were problems (2 sub-exceptions)
>>>
```
By using `except*` instead of `except`, we can selectively handle only the exceptions in the group that match a certain type. In the following example, which shows a nested exception group, each `except*` clause extracts from the group exceptions of a certain type while letting all other exceptions propagate to other clauses and eventually to be reraised.
Copy
```
>>> def f():
... raise ExceptionGroup(
... "group1",
... [
... OSError(1),
... SystemError(2),
... ExceptionGroup(
... "group2",
... [
... OSError(3),
... RecursionError(4)
... ]
... )
... ]
... )
...
>>> try:
... f()
... except* OSError as e:
... print("There were OSErrors")
... except* SystemError as e:
... print("There were SystemErrors")
...
There were OSErrors
There were SystemErrors
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 2, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise ExceptionGroup(
| ...<12 lines>...
| )
| ExceptionGroup: group1 (1 sub-exception)
+-+---------------- 1 ----------------
| ExceptionGroup: group2 (1 sub-exception)
+-+---------------- 1 ----------------
| RecursionError: 4
+------------------------------------
>>>
```
Note that the exceptions nested in an exception group must be instances, not types. This is because in practice the exceptions would typically be ones that have already been raised and caught by the program, along the following pattern:
Copy
```
>>> excs = []
... for test in tests:
... try:
... test.run()
... except Exception as e:
... excs.append(e)
...
>>> if excs:
... raise ExceptionGroup("Test Failures", excs)
...
```
## 8\.10. Enriching Exceptions with Notes[¶](https://docs.python.org/3/tutorial/errors.html#enriching-exceptions-with-notes "Link to this heading")
When an exception is created in order to be raised, it is usually initialized with information that describes the error that has occurred. There are cases where it is useful to add information after the exception was caught. For this purpose, exceptions have a method `add_note(note)` that accepts a string and adds it to the exceptionâs notes list. The standard traceback rendering includes all notes, in the order they were added, after the exception.
Copy
```
>>> try:
... raise TypeError('bad type')
... except Exception as e:
... e.add_note('Add some information')
... e.add_note('Add some more information')
... raise
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
raise TypeError('bad type')
TypeError: bad type
Add some information
Add some more information
>>>
```
For example, when collecting exceptions into an exception group, we may want to add context information for the individual errors. In the following each exception in the group has a note indicating when this error has occurred.
Copy
```
>>> def f():
... raise OSError('operation failed')
...
>>> excs = []
>>> for i in range(3):
... try:
... f()
... except Exception as e:
... e.add_note(f'Happened in Iteration {i+1}')
... excs.append(e)
...
>>> raise ExceptionGroup('We have some problems', excs)
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 1, in <module>
| raise ExceptionGroup('We have some problems', excs)
| ExceptionGroup: We have some problems (3 sub-exceptions)
+-+---------------- 1 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 1
+---------------- 2 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 2
+---------------- 3 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 3
+------------------------------------
>>>
```
### [Table of Contents](https://docs.python.org/3/contents.html)
- [8\. Errors and Exceptions](https://docs.python.org/3/tutorial/errors.html)
- [8\.1. Syntax Errors](https://docs.python.org/3/tutorial/errors.html#syntax-errors)
- [8\.2. Exceptions](https://docs.python.org/3/tutorial/errors.html#exceptions)
- [8\.3. Handling Exceptions](https://docs.python.org/3/tutorial/errors.html#handling-exceptions)
- [8\.4. Raising Exceptions](https://docs.python.org/3/tutorial/errors.html#raising-exceptions)
- [8\.5. Exception Chaining](https://docs.python.org/3/tutorial/errors.html#exception-chaining)
- [8\.6. User-defined Exceptions](https://docs.python.org/3/tutorial/errors.html#user-defined-exceptions)
- [8\.7. Defining Clean-up Actions](https://docs.python.org/3/tutorial/errors.html#defining-clean-up-actions)
- [8\.8. Predefined Clean-up Actions](https://docs.python.org/3/tutorial/errors.html#predefined-clean-up-actions)
- [8\.9. Raising and Handling Multiple Unrelated Exceptions](https://docs.python.org/3/tutorial/errors.html#raising-and-handling-multiple-unrelated-exceptions)
- [8\.10. Enriching Exceptions with Notes](https://docs.python.org/3/tutorial/errors.html#enriching-exceptions-with-notes)
#### Previous topic
[7\. Input and Output](https://docs.python.org/3/tutorial/inputoutput.html "previous chapter")
#### Next topic
[9\. Classes](https://docs.python.org/3/tutorial/classes.html "next chapter")
### This page
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| Readable Markdown | Until now error messages havenât been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: *syntax errors* and *exceptions*.
## 8\.1. Syntax Errors[¶](https://docs.python.org/3/tutorial/errors.html#syntax-errors "Link to this heading")
Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python:
```
>>> while True print('Hello world')
File "<stdin>", line 1
while True print('Hello world')
^^^^^
SyntaxError: invalid syntax
```
The parser repeats the offending line and displays little arrows pointing at the place where the error was detected. Note that this is not always the place that needs to be fixed. In the example, the error is detected at the function [`print()`](https://docs.python.org/3/library/functions.html#print "print"), since a colon (`':'`) is missing just before it.
The file name (`<stdin>` in our example) and line number are printed so you know where to look in case the input came from a file.
## 8\.2. Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#exceptions "Link to this heading")
Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called *exceptions* and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here:
```
>>> 10 * (1/0)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
10 * (1/0)
~^~
ZeroDivisionError: division by zero
>>> 4 + spam*3
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
4 + spam*3
^^^^
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
'2' + 2
~~~~^~~
TypeError: can only concatenate str (not "int") to str
```
The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are [`ZeroDivisionError`](https://docs.python.org/3/library/exceptions.html#ZeroDivisionError "ZeroDivisionError"), [`NameError`](https://docs.python.org/3/library/exceptions.html#NameError "NameError") and [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError"). The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords).
The rest of the line provides detail based on the type of exception and what caused it.
The preceding part of the error message shows the context where the exception occurred, in the form of a stack traceback. In general it contains a stack traceback listing source lines; however, it will not display lines read from standard input.
[Built-in Exceptions](https://docs.python.org/3/library/exceptions.html#bltin-exceptions) lists the built-in exceptions and their meanings.
## 8\.3. Handling Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#handling-exceptions "Link to this heading")
It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using `Control`\-`C` or whatever the operating system supports); note that a user-generated interruption is signalled by raising the [`KeyboardInterrupt`](https://docs.python.org/3/library/exceptions.html#KeyboardInterrupt "KeyboardInterrupt") exception.
```
>>> while True:
... try:
... x = int(input("Please enter a number: "))
... break
... except ValueError:
... print("Oops! That was no valid number. Try again...")
...
```
The [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement works as follows.
- First, the *try clause* (the statement(s) between the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) and [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) keywords) is executed.
- If no exception occurs, the *except clause* is skipped and execution of the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement is finished.
- If an exception occurs during execution of the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) clause, the rest of the clause is skipped. Then, if its type matches the exception named after the [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) keyword, the *except clause* is executed, and then execution continues after the try/except block.
- If an exception occurs which does not match the exception named in the *except clause*, it is passed on to outer [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statements; if no handler is found, it is an *unhandled exception* and execution stops with an error message.
A [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement may have more than one *except clause*, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding *try clause*, not in other handlers of the same `try` statement. An *except clause* may name multiple exceptions, for example:
```
... except RuntimeError, TypeError, NameError:
... pass
```
A class in an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) clause matches exceptions which are instances of the class itself or one of its derived classes (but not the other way around â an *except clause* listing a derived class does not match instances of its base classes). For example, the following code will print B, C, D in that order:
```
class B(Exception):
pass
class C(B):
pass
class D(C):
pass
for cls in [B, C, D]:
try:
raise cls()
except D:
print("D")
except C:
print("C")
except B:
print("B")
```
Note that if the *except clauses* were reversed (with `except B` first), it would have printed B, B, B â the first matching *except clause* is triggered.
When an exception occurs, it may have associated values, also known as the exceptionâs *arguments*. The presence and types of the arguments depend on the exception type.
The *except clause* may specify a variable after the exception name. The variable is bound to the exception instance which typically has an `args` attribute that stores the arguments. For convenience, builtin exception types define [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__") to print all the arguments without explicitly accessing `.args`.
```
>>> try:
... raise Exception('spam', 'eggs')
... except Exception as inst:
... print(type(inst)) # the exception type
... print(inst.args) # arguments stored in .args
... print(inst) # __str__ allows args to be printed directly,
... # but may be overridden in exception subclasses
... x, y = inst.args # unpack args
... print('x =', x)
... print('y =', y)
...
<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs
```
The exceptionâs [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__") output is printed as the last part (âdetailâ) of the message for unhandled exceptions.
[`BaseException`](https://docs.python.org/3/library/exceptions.html#BaseException "BaseException") is the common base class of all exceptions. One of its subclasses, [`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception"), is the base class of all the non-fatal exceptions. Exceptions which are not subclasses of `Exception` are not typically handled, because they are used to indicate that the program should terminate. They include [`SystemExit`](https://docs.python.org/3/library/exceptions.html#SystemExit "SystemExit") which is raised by [`sys.exit()`](https://docs.python.org/3/library/sys.html#sys.exit "sys.exit") and [`KeyboardInterrupt`](https://docs.python.org/3/library/exceptions.html#KeyboardInterrupt "KeyboardInterrupt") which is raised when a user wishes to interrupt the program.
[`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception") can be used as a wildcard that catches (almost) everything. However, it is good practice to be as specific as possible with the types of exceptions that we intend to handle, and to allow any unexpected exceptions to propagate on.
The most common pattern for handling [`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception") is to print or log the exception and then re-raise it (allowing a caller to handle the exception as well):
```
import sys
try:
f = open('myfile.txt')
s = f.readline()
i = int(s.strip())
except OSError as err:
print("OS error:", err)
except ValueError:
print("Could not convert data to an integer.")
except Exception as err:
print(f"Unexpected {err=}, {type(err)=}")
raise
```
The [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) ⊠[`except`](https://docs.python.org/3/reference/compound_stmts.html#except) statement has an optional *else clause*, which, when present, must follow all *except clauses*. It is useful for code that must be executed if the *try clause* does not raise an exception. For example:
```
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except OSError:
print('cannot open', arg)
else:
print(arg, 'has', len(f.readlines()), 'lines')
f.close()
```
The use of the `else` clause is better than adding additional code to the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) clause because it avoids accidentally catching an exception that wasnât raised by the code being protected by the `try` ⊠`except` statement.
Exception handlers do not handle only exceptions that occur immediately in the *try clause*, but also those that occur inside functions that are called (even indirectly) in the *try clause*. For example:
```
>>> def this_fails():
... x = 1/0
...
>>> try:
... this_fails()
... except ZeroDivisionError as err:
... print('Handling run-time error:', err)
...
Handling run-time error: division by zero
```
## 8\.4. Raising Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#raising-exceptions "Link to this heading")
The [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) statement allows the programmer to force a specified exception to occur. For example:
```
>>> raise NameError('HiThere')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
raise NameError('HiThere')
NameError: HiThere
```
The sole argument to [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from [`BaseException`](https://docs.python.org/3/library/exceptions.html#BaseException "BaseException"), such as [`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception") or one of its subclasses). If an exception class is passed, it will be implicitly instantiated by calling its constructor with no arguments:
```
raise ValueError # shorthand for 'raise ValueError()'
```
If you need to determine whether an exception was raised but donât intend to handle it, a simpler form of the [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) statement allows you to re-raise the exception:
```
>>> try:
... raise NameError('HiThere')
... except NameError:
... print('An exception flew by!')
... raise
...
An exception flew by!
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
raise NameError('HiThere')
NameError: HiThere
```
## 8\.5. Exception Chaining[¶](https://docs.python.org/3/tutorial/errors.html#exception-chaining "Link to this heading")
If an unhandled exception occurs inside an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) section, it will have the exception being handled attached to it and included in the error message:
```
>>> try:
... open("database.sqlite")
... except OSError:
... raise RuntimeError("unable to handle error")
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
open("database.sqlite")
~~~~^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'database.sqlite'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError("unable to handle error")
RuntimeError: unable to handle error
```
To indicate that an exception is a direct consequence of another, the [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) statement allows an optional `from` clause:
```
# exc must be exception instance or None.
raise RuntimeError from exc
```
This can be useful when you are transforming exceptions. For example:
```
>>> def func():
... raise ConnectionError
...
>>> try:
... func()
... except ConnectionError as exc:
... raise RuntimeError('Failed to open database') from exc
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
func()
~~~~^^
File "<stdin>", line 2, in func
ConnectionError
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError('Failed to open database') from exc
RuntimeError: Failed to open database
```
It also allows disabling automatic exception chaining using the `from None` idiom:
```
>>> try:
... open('database.sqlite')
... except OSError:
... raise RuntimeError from None
...
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError from None
RuntimeError
```
For more information about chaining mechanics, see [Built-in Exceptions](https://docs.python.org/3/library/exceptions.html#bltin-exceptions).
## 8\.6. User-defined Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#user-defined-exceptions "Link to this heading")
Programs may name their own exceptions by creating a new exception class (see [Classes](https://docs.python.org/3/tutorial/classes.html#tut-classes) for more about Python classes). Exceptions should typically be derived from the [`Exception`](https://docs.python.org/3/library/exceptions.html#Exception "Exception") class, either directly or indirectly.
Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception.
Most exceptions are defined with names that end in âErrorâ, similar to the naming of the standard exceptions.
Many standard modules define their own exceptions to report errors that may occur in functions they define.
## 8\.7. Defining Clean-up Actions[¶](https://docs.python.org/3/tutorial/errors.html#defining-clean-up-actions "Link to this heading")
The [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:
```
>>> try:
... raise KeyboardInterrupt
... finally:
... print('Goodbye, world!')
...
Goodbye, world!
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
raise KeyboardInterrupt
KeyboardInterrupt
```
If a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause is present, the `finally` clause will execute as the last task before the [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement completes. The `finally` clause runs whether or not the `try` statement produces an exception. The following points discuss more complex cases when an exception occurs:
- If an exception occurs during execution of the `try` clause, the exception may be handled by an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) clause. If the exception is not handled by an `except` clause, the exception is re-raised after the `finally` clause has been executed.
- An exception could occur during execution of an `except` or `else` clause. Again, the exception is re-raised after the `finally` clause has been executed.
- If the `finally` clause executes a [`break`](https://docs.python.org/3/reference/simple_stmts.html#break), [`continue`](https://docs.python.org/3/reference/simple_stmts.html#continue) or [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement, exceptions are not re-raised. This can be confusing and is therefore discouraged. From version 3.14 the compiler emits a [`SyntaxWarning`](https://docs.python.org/3/library/exceptions.html#SyntaxWarning "SyntaxWarning") for it (see [**PEP 765**](https://peps.python.org/pep-0765/)).
- If the `try` statement reaches a [`break`](https://docs.python.org/3/reference/simple_stmts.html#break), [`continue`](https://docs.python.org/3/reference/simple_stmts.html#continue) or [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement, the `finally` clause will execute just prior to the `break`, `continue` or `return` statementâs execution.
- If a `finally` clause includes a `return` statement, the returned value will be the one from the `finally` clauseâs `return` statement, not the value from the `try` clauseâs `return` statement. This can be confusing and is therefore discouraged. From version 3.14 the compiler emits a [`SyntaxWarning`](https://docs.python.org/3/library/exceptions.html#SyntaxWarning "SyntaxWarning") for it (see [**PEP 765**](https://peps.python.org/pep-0765/)).
For example:
```
>>> def bool_return():
... try:
... return True
... finally:
... return False
...
>>> bool_return()
False
```
A more complicated example:
```
>>> def divide(x, y):
... try:
... result = x / y
... except ZeroDivisionError:
... print("division by zero!")
... else:
... print("result is", result)
... finally:
... print("executing finally clause")
...
>>> divide(2, 1)
result is 2.0
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
divide("2", "1")
~~~~~~^^^^^^^^^^
File "<stdin>", line 3, in divide
result = x / y
~~^~~
TypeError: unsupported operand type(s) for /: 'str' and 'str'
```
As you can see, the [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause is executed in any event. The [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") raised by dividing two strings is not handled by the [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) clause and therefore re-raised after the `finally` clause has been executed.
In real world applications, the [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.
## 8\.8. Predefined Clean-up Actions[¶](https://docs.python.org/3/tutorial/errors.html#predefined-clean-up-actions "Link to this heading")
Some objects define standard clean-up actions to be undertaken when the object is no longer needed, regardless of whether or not the operation using the object succeeded or failed. Look at the following example, which tries to open a file and print its contents to the screen.
```
for line in open("myfile.txt"):
print(line, end="")
```
The problem with this code is that it leaves the file open for an indeterminate amount of time after this part of the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. The [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.
```
with open("myfile.txt") as f:
for line in f:
print(line, end="")
```
After the statement is executed, the file *f* is always closed, even if a problem was encountered while processing the lines. Objects which, like files, provide predefined clean-up actions will indicate this in their documentation.
## 8\.9. Raising and Handling Multiple Unrelated Exceptions[¶](https://docs.python.org/3/tutorial/errors.html#raising-and-handling-multiple-unrelated-exceptions "Link to this heading")
There are situations where it is necessary to report several exceptions that have occurred. This is often the case in concurrency frameworks, when several tasks may have failed in parallel, but there are also other use cases where it is desirable to continue execution and collect multiple errors rather than raise the first exception.
The builtin [`ExceptionGroup`](https://docs.python.org/3/library/exceptions.html#ExceptionGroup "ExceptionGroup") wraps a list of exception instances so that they can be raised together. It is an exception itself, so it can be caught like any other exception.
```
>>> def f():
... excs = [OSError('error 1'), SystemError('error 2')]
... raise ExceptionGroup('there were problems', excs)
...
>>> f()
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 1, in <module>
| f()
| ~^^
| File "<stdin>", line 3, in f
| raise ExceptionGroup('there were problems', excs)
| ExceptionGroup: there were problems (2 sub-exceptions)
+-+---------------- 1 ----------------
| OSError: error 1
+---------------- 2 ----------------
| SystemError: error 2
+------------------------------------
>>> try:
... f()
... except Exception as e:
... print(f'caught {type(e)}: {e}')
...
caught <class 'ExceptionGroup'>: there were problems (2 sub-exceptions)
>>>
```
By using `except*` instead of `except`, we can selectively handle only the exceptions in the group that match a certain type. In the following example, which shows a nested exception group, each `except*` clause extracts from the group exceptions of a certain type while letting all other exceptions propagate to other clauses and eventually to be reraised.
```
>>> def f():
... raise ExceptionGroup(
... "group1",
... [
... OSError(1),
... SystemError(2),
... ExceptionGroup(
... "group2",
... [
... OSError(3),
... RecursionError(4)
... ]
... )
... ]
... )
...
>>> try:
... f()
... except* OSError as e:
... print("There were OSErrors")
... except* SystemError as e:
... print("There were SystemErrors")
...
There were OSErrors
There were SystemErrors
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 2, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise ExceptionGroup(
| ...<12 lines>...
| )
| ExceptionGroup: group1 (1 sub-exception)
+-+---------------- 1 ----------------
| ExceptionGroup: group2 (1 sub-exception)
+-+---------------- 1 ----------------
| RecursionError: 4
+------------------------------------
>>>
```
Note that the exceptions nested in an exception group must be instances, not types. This is because in practice the exceptions would typically be ones that have already been raised and caught by the program, along the following pattern:
```
>>> excs = []
... for test in tests:
... try:
... test.run()
... except Exception as e:
... excs.append(e)
...
>>> if excs:
... raise ExceptionGroup("Test Failures", excs)
...
```
## 8\.10. Enriching Exceptions with Notes[¶](https://docs.python.org/3/tutorial/errors.html#enriching-exceptions-with-notes "Link to this heading")
When an exception is created in order to be raised, it is usually initialized with information that describes the error that has occurred. There are cases where it is useful to add information after the exception was caught. For this purpose, exceptions have a method `add_note(note)` that accepts a string and adds it to the exceptionâs notes list. The standard traceback rendering includes all notes, in the order they were added, after the exception.
```
>>> try:
... raise TypeError('bad type')
... except Exception as e:
... e.add_note('Add some information')
... e.add_note('Add some more information')
... raise
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
raise TypeError('bad type')
TypeError: bad type
Add some information
Add some more information
>>>
```
For example, when collecting exceptions into an exception group, we may want to add context information for the individual errors. In the following each exception in the group has a note indicating when this error has occurred.
```
>>> def f():
... raise OSError('operation failed')
...
>>> excs = []
>>> for i in range(3):
... try:
... f()
... except Exception as e:
... e.add_note(f'Happened in Iteration {i+1}')
... excs.append(e)
...
>>> raise ExceptionGroup('We have some problems', excs)
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 1, in <module>
| raise ExceptionGroup('We have some problems', excs)
| ExceptionGroup: We have some problems (3 sub-exceptions)
+-+---------------- 1 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 1
+---------------- 2 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 2
+---------------- 3 ----------------
| Traceback (most recent call last):
| File "<stdin>", line 3, in <module>
| f()
| ~^^
| File "<stdin>", line 2, in f
| raise OSError('operation failed')
| OSError: operation failed
| Happened in Iteration 3
+------------------------------------
>>>
``` |
| Shard | 16 (laksa) |
| Root Hash | 10954876678907435016 |
| Unparsed URL | org,python!docs,/3/tutorial/errors.html s443 |