Introduction

A Metaclass means the class of a class. In Python Classes are objects, which are instances of some Class.

As can see below every Class in Python are instance of a class type.

class Example:
    pass

>>> type(Example)
 <class 'type'>

type is a MetaClass in Python which is responsible for classes creation.

Dynamic Class creation using type()

  1. When type() is called with one argument, it returns the argument type.

  2. When type() is called with 3 arguments, a class is created.

    • Class name
    • Tuple having base classes inherited by class
    • Class dictionary populated with class methods and variables
# creating a base class
class Base:
    def myfunc(self):
        print("This is from Base!")

# Normal Way
class Test(Base):
    x = 1

# Dynamic Way using type()
Test = type('Test', (Base, ), dict(x=1))

Creating Custom Metaclass

Let’s consider an example:

class Foo:
    pass

>>> f = Foo()

The expression Foo() creates a new instance of class Foo and the following occurs:

  • The __call__ method of Foo’s parent class is called. Since Foo is a standard new-style class, its parent class is the type metaclass, so type’s __call__ method is invoked.

  • That __call__ method in turn invokes the following:

    • __new__
    • __init__

To create a Custom MetaClass, the class has to inherit type and overide __new__, __init__ methods.

Notes

  1. Metaclasses can be used when the classes needs to change dynamically.
  2. Metaclasses can be used when a decorator needs to be applied to every function in a class.