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What is a Python metaclass?

Educative Answers Team

Classes are used to make objects. However, a class itself is an object and, just like any other object, it’s an instance of something: a ​metaclass.

A metaclass is the class of a class; it defines how a class behaves.

Hence, if we want to modify the behavior of classes, we will need to write our own custom metaclass.

The default metaclass is type. This means that just like an object is of the type class, a class itself is of the type, type.

svg viewer
class Foobar:
  pass
foo = Foobar()

print(type(foo))
print(type(Foobar))

Running the code extract above​ shows us that since foo is an instantiation of the class Foobar, its type is the class Foobar. The class Foobar, on the other hand, displays its own type as the class type, which, as we earlier said, is the default metaclass.

Defining custom metaclasses

The type() function can be used to directly define classes by passing it through:

  • the class name.
  • the tuple of the base classes inherited by the class.
  • a dictionary containing the class’ methods and variables.
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​To create a custom metaclass, we first have to inherit the default metaclass, type, and then override its __new__() and __init__() methods. The following code defines a metaclass with a custom __new__() method which defines a new attribute x in the class. The Foo class inherits the attributes of its metaclass Meta:

class Meta(type):
    # overriding the new method of the metaclass
    def __new__(cls, name, bases, dct):
        x = super().__new__(cls, name, bases, dct)
        # defining that each class with th=is metaclass
        # should have a variable, x with a default value
        x.attr = 100
        return x

# defining a class with Meta as its metaclass instead
# of type
class Foo(metaclass = Meta):
    pass

# printing the variables in our newly defined class
print(Foo.attr)

Uses of custom metaclasses

Metaclasses can be applied in logging, registration of classes at creation time, profiling, and others. They propagate down the inheritance hierarchies, allowing all subclasses to inherit their attributes and methods. This property is particularly useful if we want our classes to have specific attributes, automatically, at the time of their creation.

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