What are metaclasses in Python?

Better Stack Team

Updated on January 24, 2023

In Python, a metaclass is the class of a class. It defines how a class behaves, including how it is created and how it manages its instances. A metaclass is defined by inheriting from the built-in type class, and it can define methods that customize the behavior of the class it creates. The class that a metaclass creates is called its "metaclass instance" or simply its "instance." One use of metaclasses is to create singleton classes, classes with only one instance.

Here's an example of a simple metaclass that creates a singleton class:

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class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]

class MySingletonClass(metaclass=Singleton):
    pass

a = MySingletonClass()
b = MySingletonClass()
print(a is b)  # True

Here, the Singleton metaclass defines a __call__ method that intercepts the creation of new instances of the class it creates. It keeps track of instances it has created in the _instances dictionary, and if an instance of the class already exists, it returns that instance instead of creating a new one. So, every time you create an instance of MySingletonClass, you get the same object.

This example is just a simple illustration of what metaclasses can do, and in most cases, this type of requirement could also be achieved by using other design patterns or python inbuilt modules like Singleton from module singleton-decorator` .

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