__subclasscheck__ and __subclasshook__ in Python

Class is a collection of data (variables and methods). It bundles data and functionality together. It provides all standard features of object-oriented programming. Basically it is a blueprint for creating objects. Creating a new class creates a new type of object, allowing new instances of that type to be made.

Example:

# class 'A' defined
class A(object):

    # Calling Constructor
    def __init__(self, a):

        self.a = a
        print("The value of a:", self.a)

# Driver's code
c = A(7)

Output:

The value of a: 7

__subclasscheck__ in Python

__subclasscheck__ is one of the methods to customize the result of issubclass() built-in function. It is a method to check whether a class is a subclass or not and returns True if the class is considered as a subclass(direct or indirect) of another class, otherwise, returns False. It cannot be defined as a class method in the actual/real class. It is implemented in the metaclass, as it is not for ordinary classes. Consider the below example for better understanding.

Example: Consider a situation where you want to check if a certain value is present as an attribute inside a class using the issubclass() method.

# Python program to demonstrate
# subclasscheck


class A(type):

    # __subclasscheck__() defined
    def __subclasscheck__(cls, sub):

        # Getting the L attribute of
        # subclass
        attr = getattr(cls, 'L', [])

        # Checking if the subclass
        # is present in the L attribute
        # of subclass or not
        if sub in attr:
            return True

        return False


class B(metaclass = A):

    # L Attribute
    L = [1, 2, 3, 4, 5]

class C(metaclass = A):

    # L Attribute
    L = ["Geeks", "For"]


# Driver's code
print(issubclass(1, B))
print(issubclass("Geeks", B))
print(issubclass("Geeks", C))

Output:

True
False
True

__subclasshook__ in Python

Abstract class can override __subclasshook__() method to customize issubclass(). It returns True when a class is found to be subclass of a ABC class, it returns False if it is not and returns NotImplemented if the subclass check is continued with the usual mechanism. This method is defined in the ABC class with some conditions. Classes that follow those conditions are considered to be a subclass.

Note: It must be defined as a class method.

Example:

# Python program to demonstrate
# subclasshook


from abc import ABCMeta


class A(metaclass = ABCMeta):

    @classmethod
    def __subclasshook__(cls, C):
        if cls is A:

            # condition to check if the
            # function anyfun() is present
            # in any sub class or not
            if any("__anyfun__" in Q.__dict__
                   for Q in C.__mro__):
                return True

        return False

class P(object):
    pass

class Q(object):

    def __anyfun__(self):
        return 0


# Driver's code
print(issubclass(Q, A) )
print(issubclass(P, A)  )

Output:

True
False