Python type Class

Summary: in this tutorial, you’ll learn about the Python type class and understand how Python uses the type class to create other classes.

Introduction to the Python type class

In Python, a class is an object of the class type. For example, the following defines the Person class with two methods __init__ and greeting:

class Person: def __init__(self, name, age): self.name = name self.age = age def greeting(self): return f'Hi, I am {self.name}. I am {self.age} year old.'
Code language: Python (python)

The Person class is an object of the class type as shown in the following statement:

print(type(Person))
Code language: Python (python)

Output:

<class 'type'>
Code language: Python (python)

The Person class is an instance of the type class:

print(isinstance(Person, type))
Code language: Python (python)

Python uses the type class to create other classes. The type class itself is a callable. The following shows one of the constructors of the type class:

type(name, bases, dict) -> a new type
Code language: Python (python)

The constructor has three parameters for creating a new class:

  • name: is the name of the class e.g., Person
  • bases is a tuple that contains the base classes of the new class. For example, the Person inherits from the object class, so the bases contains one class (object,)
  • dict is the class namespace

Technically, you can use the type class to create a class dynamically. Before doing it, you need to understand how Python creates the classes.

When the Python interpreter encounters a class definition in the code, it will:

  • First, extract the class body as string.
  • Second, create a class dictionary for the class namespace.
  • Third, execute the class body to fill up the class dictionary.
  • Finally, create a new instance of type using the above type() constructor.

Let’s emulate the steps above to create a Person class dynamically:

First, extract the class body:

class_body = """ def __init__(self, name, age): self.name = name self.age = age def greeting(self): return f'Hi, I am {self.name}. I am {self.age} year old.' """
Code language: Python (python)

Second, create a class dictionary:

class_dict = {}
Code language: Python (python)

Third, execute the class body and fill up the class dictionary:

exec(class_body, globals(), class_dict)
Code language: Python (python)

The exec() function executes the class body and fills up the global and class namespaces.

Finally, create a new Person class using the type constructor:

Person = type('Person', (object,), class_dict)
Code language: Python (python)

Note that the Person is a class, which is also an object. The Person class inherits from the object class and has the namespace of the class_dict.

The following shows the type of the Person class which is the type class:

<class 'type'>
Code language: Python (python)

And it is an instance of the type class:

print(isinstance(Person, type))
Code language: Python (python)

Output:

True
Code language: Python (python)

The class_dict has the two functions __init__ and greeting:

{'__init__': <function __init__ at 0x0000024E28465160>, 'greeting': <function greeting at 0x0000024E28931550>}
Code language: Python (python)

The Person.__dict__ also contains these functions :

mappingproxy({'__dict__': <attribute '__dict__' of 'Person' objects>, '__doc__': None, '__init__': <function __init__ at 0x00000165F7725160>, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'Person' objects>, 'greeting': <function greeting at 0x00000165F7F71550>})
Code language: Python (python)

In this example, Python creates the type class dynamically, which is the same as the one that you define statically in the code.

Because the type class creates other classes, we often refer to it as a metaclass. A metaclass is a class used to create other classes.

Summary

  • In Python, a class is an instance of the type class.
  • The type class creates other classs, therefore, it is called a metaclass.
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