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[Home](https://www.datacamp.com/) 2. [Tutorials](https://www.datacamp.com/tutorial) 3. [Python](https://www.datacamp.com/tutorial/category/python) # Introduction to Python Metaclasses In this tutorial, learn what metaclasses are, how to implement them in Python, and how to create custom ones. Contents Dec 10, 2018 · 6 min read Contents - [Everything in Python is an Object](https://www.datacamp.com/tutorial/python-metaclasses#everything-in-python-is-an-object-<code) - [Python Classes can be Created Dynamically](https://www.datacamp.com/tutorial/python-metaclasses#python-classes-can-be-created-dynamically-<code) - [Creating Custom Metaclasses](https://www.datacamp.com/tutorial/python-metaclasses#creating-custom-metaclasses-inpyt) - [\_\_new\_\_ and \_\_init\_\_](https://www.datacamp.com/tutorial/python-metaclasses#__new__-and-__init__-metac) - [Metaclass \_\_call\_\_ method](https://www.datacamp.com/tutorial/python-metaclasses#metaclass-__call__-method-accor) - [Metaclass \_\_prepare\_\_ method](https://www.datacamp.com/tutorial/python-metaclasses#metaclass-__prepare__-method-accor) - [Singleton Design using a Metaclass](https://www.datacamp.com/tutorial/python-metaclasses#singleton-design-using-a-metaclass-thisi) - [Conclusion](https://www.datacamp.com/tutorial/python-metaclasses#conclusion-inthi) - [References](https://www.datacamp.com/tutorial/python-metaclasses#references-<ahre) ## Training more people? Get your team access to the full DataCamp for business platform. [For Business](https://www.datacamp.com/business)For a bespoke solution [book a demo](https://www.datacamp.com/business/demo-2). A metaclass in [Python](https://www.datacamp.com/tutorial/python) is a class of a class that defines how a class behaves. A class is itself an instance of a metaclass. A class in Python defines how the instance of the class will behave. In order to understand metaclasses well, one needs to have prior experience working with Python classes. Before we dive deeper into metaclasses, let's get a few concepts out of the way. ### Everything in Python is an Object ``` Powered ByWas this AI assistant helpful? Yes No ``` ``` <__main__.TestClass object at 0x7f6fcc6bf908>Powered ByWas this AI assistant helpful? Yes No ``` ### Python Classes can be Created Dynamically `type` in Python enables us to find the type of an object. We can proceed to check the type of object we created above. ``` type(TestClass)Powered ByWas this AI assistant helpful? Yes No ``` ``` typePowered ByWas this AI assistant helpful? Yes No ``` ``` type(type)Powered ByWas this AI assistant helpful? Yes No ``` ``` typePowered ByWas this AI assistant helpful? Yes No ``` Wait, What just happened? We'd expect the type of the object we created above to be class, but it's not. Hold on to that thought. We will cover it further in a few. We also notice that the type of `type` itself is `type`. It is an instance of `type`. Another magical thing that `type` does is enable us to create classes dynamically. Let's show how we'd do that below. The `DataCamp` class shown below would be created as shown below using `type`: ``` Powered ByWas this AI assistant helpful? Yes No ``` ``` Powered ByWas this AI assistant helpful? Yes No ``` ``` Powered ByWas this AI assistant helpful? Yes No ``` In the above example `DataCamp` is the class name while `DataCampClass` is the variable that holds the class reference. When using `type` we can pass attributes of the class using a dictionary as shown below: ``` Powered ByWas this AI assistant helpful? Yes No ``` ``` Powered ByWas this AI assistant helpful? Yes No ``` In case we wanted our `PythonClass` to inherit from the `DataCamp` class we pass it to our second argument when defining the class using `type` ``` PythonClass = type('PythonClass', (DataCamp,), {'start_date': 'August 2018', 'instructor': 'John Doe'} )Powered ByWas this AI assistant helpful? Yes No ``` ``` print(PythonClass)Powered ByWas this AI assistant helpful? Yes No ``` ``` <class '__main__.PythonClass'>Powered ByWas this AI assistant helpful? Yes No ``` Now that those two concepts are out of the way, we realize that Python creates the classes using a metaclass. We have seen that everything in Python is an object, these objects are created by metaclasses. Whenever we call `class` to create a class, there is a metaclass that does the magic of creating the class behind the scenes. We've already seen `type` do this in practice above. It is similar to `str` that creates strings and `int` that creates integers. In Python, the `___class__`attribute enables us to check the type of the current instance. Let's create a string below and check its type. ``` Powered ByWas this AI assistant helpful? Yes No ``` ``` strPowered ByWas this AI assistant helpful? Yes No ``` We can also check the type using `type(article)`. ``` type(article)Powered ByWas this AI assistant helpful? Yes No ``` ``` strPowered ByWas this AI assistant helpful? Yes No ``` When we check the type of `str`, we also find out that it's type. ``` type(str)Powered ByWas this AI assistant helpful? Yes No ``` ``` typePowered ByWas this AI assistant helpful? Yes No ``` When we check the type for `float`, `int`, `list`, `tuple`, and `dict`, we will have a similar output. This is because all of these objects are of type `type`. ``` print(type(list),type(float), type(dict), type(tuple))Powered ByWas this AI assistant helpful? Yes No ``` ``` <class 'type'> <class 'type'> <class 'type'> <class 'type'>Powered ByWas this AI assistant helpful? Yes No ``` We've already seen `type` creates classes. Hence when we check the `__class__` of `__class__` it should return `type`. ``` article.__class__.__class__Powered ByWas this AI assistant helpful? Yes No ``` ``` typePowered ByWas this AI assistant helpful? Yes No ``` ## Master your data skills with DataCamp More than 10 million people learn Python, R, SQL, and other tech skills using our hands-on courses crafted by industry experts. [Start Learning](https://www.datacamp.com/courses-all)  ## Creating Custom Metaclasses In Python, we can customize the class creation process by passing the `metaclass` keyword in the class definition. This can also be done by inheriting a class that has already passed in this keyword. ``` Powered ByWas this AI assistant helpful? Yes No ``` We can see below that the type of `MyMeta` class is `type` and that the type of `MyClass` and `MySubClass` is `MyMeta`. ``` Powered ByWas this AI assistant helpful? Yes No ``` ``` Powered ByWas this AI assistant helpful? Yes No ``` When defining a class and no metaclass is defined the default `type` metaclass will be used. If a metaclass is given and it is not an instance of `type()`, then it is used directly as the metaclass. ## `__new__` and `__init__` Metaclasses can also be defined in one of the two ways shown below. We'll explain the difference between them below. ``` Powered ByWas this AI assistant helpful? Yes No ``` `__new__` is used when one wants to define dict or bases tuples before the class is created. The return value of `__new__`is usually an instance of `cls`. `__new__` allows subclasses of immutable types to customize instance creation. It can be overridden in custom metaclasses to customize class creation. `__init__` is usually called after the object has been created so as to initialize it. ### Metaclass `__call__` method According to the official [docs](https://docs.python.org/3/reference/datamodel.html), we can also override other class methods by defining a custom `__call__()` method in the metaclass that allows custom behavior when the class is called. ### Metaclass `__prepare__` method According to Python's [data model docs](https://docs.python.org/3/reference/datamodel.html) *Once the appropriate metaclass has been identified, then the class namespace is prepared. If the metaclass has a `__prepare__` attribute, it is called as `namespace = metaclass.__prepare__(name, bases, **kwds)` (where the additional keyword arguments, if any, come from the class definition). If the metaclass has no `__prepare__`attribute, then the class namespace is initialized as an empty ordered mapping.* - [docs.python.org](https://docs.python.org/3/reference/datamodel.html?highlight=__setattr__) ### Singleton Design using a Metaclass This is a design pattern that restricts the instantiation of a class to only one object. This could prove useful for example when designing a class to connect to the database. One might want to have just one instance of the connection class. ``` Powered ByWas this AI assistant helpful? Yes No ``` ## Conclusion In this article, we have learned about what metaclasses are and how we can implement them in our Python programming. Metaclasses can be applied in logging, registration of classes at creation time and profiling among others. 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A metaclass in [Python](https://www.datacamp.com/tutorial/python) is a class of a class that defines how a class behaves. A class is itself an instance of a metaclass. A class in Python defines how the instance of the class will behave. In order to understand metaclasses well, one needs to have prior experience working with Python classes. Before we dive deeper into metaclasses, let's get a few concepts out of the way. Everything in Python is an Object Python Classes can be Created Dynamically `type` in Python enables us to find the type of an object. We can proceed to check the type of object we created above. Wait, What just happened? We'd expect the type of the object we created above to be class, but it's not. Hold on to that thought. We will cover it further in a few. We also notice that the type of `type` itself is `type`. It is an instance of `type`. Another magical thing that `type` does is enable us to create classes dynamically. Let's show how we'd do that below. The `DataCamp` class shown below would be created as shown below using `type`: In the above example `DataCamp` is the class name while `DataCampClass` is the variable that holds the class reference. When using `type` we can pass attributes of the class using a dictionary as shown below: In case we wanted our `PythonClass` to inherit from the `DataCamp` class we pass it to our second argument when defining the class using `type` Now that those two concepts are out of the way, we realize that Python creates the classes using a metaclass. We have seen that everything in Python is an object, these objects are created by metaclasses. Whenever we call `class` to create a class, there is a metaclass that does the magic of creating the class behind the scenes. We've already seen `type` do this in practice above. It is similar to `str` that creates strings and `int` that creates integers. In Python, the `___class__`attribute enables us to check the type of the current instance. Let's create a string below and check its type. We can also check the type using `type(article)`. When we check the type of `str`, we also find out that it's type. When we check the type for `float`, `int`, `list`, `tuple`, and `dict`, we will have a similar output. This is because all of these objects are of type `type`. We've already seen `type` creates classes. Hence when we check the `__class__` of `__class__` it should return `type`. ## Master your data skills with DataCamp More than 10 million people learn Python, R, SQL, and other tech skills using our hands-on courses crafted by industry experts.  In Python, we can customize the class creation process by passing the `metaclass` keyword in the class definition. This can also be done by inheriting a class that has already passed in this keyword. We can see below that the type of `MyMeta` class is `type` and that the type of `MyClass` and `MySubClass` is `MyMeta`. When defining a class and no metaclass is defined the default `type` metaclass will be used. If a metaclass is given and it is not an instance of `type()`, then it is used directly as the metaclass. Metaclasses can also be defined in one of the two ways shown below. We'll explain the difference between them below. `__new__` is used when one wants to define dict or bases tuples before the class is created. The return value of `__new__`is usually an instance of `cls`. `__new__` allows subclasses of immutable types to customize instance creation. It can be overridden in custom metaclasses to customize class creation. `__init__` is usually called after the object has been created so as to initialize it. Metaclass `__call__` method According to the official [docs](https://docs.python.org/3/reference/datamodel.html), we can also override other class methods by defining a custom `__call__()` method in the metaclass that allows custom behavior when the class is called. Metaclass `__prepare__` method According to Python's [data model docs](https://docs.python.org/3/reference/datamodel.html) *Once the appropriate metaclass has been identified, then the class namespace is prepared. If the metaclass has a `__prepare__` attribute, it is called as `namespace = metaclass.__prepare__(name, bases, **kwds)` (where the additional keyword arguments, if any, come from the class definition). If the metaclass has no `__prepare__`attribute, then the class namespace is initialized as an empty ordered mapping.* - [docs.python.org](https://docs.python.org/3/reference/datamodel.html?highlight=__setattr__) Singleton Design using a Metaclass This is a design pattern that restricts the instantiation of a class to only one object. This could prove useful for example when designing a class to connect to the database. One might want to have just one instance of the connection class. Conclusion In this article, we have learned about what metaclasses are and how we can implement them in our Python programming. Metaclasses can be applied in logging, registration of classes at creation time and profiling among others. They seem to be quite abstract concepts, and you might be wondering if you need to use them at all. Long-term Pythonista, Tim Peters answers that question best. *"Metaclasses are deeper magic than 99% of users should ever worry about. If you wonder whether you need them, you don’t (the people who actually need them know with certainty that they need them, and don’t need an explanation about why).”* - [realpython](https://realpython.com/python-metaclasses) If you would like to learn more about Python, start DataCamp's [Python Programming](https://www.datacamp.com/tracks/python-programming) skill track and check out our [Python Classes Tutorial](https://www.datacamp.com/tutorial/python-classes). References [Link one](https://www.python.org/dev/peps/pep-3115) [Link two](https://docs.python.org/3/reference/datamodel.html#metaclasses) Topics