Previous: Refactoring in Object-Oriented Programming

There are numerous functions in Python that perform a task or calculate a result on certain types of objects without being methods on the underlying class. Some of these are closely related to class definitions, allowing us to use a functional style of programming with the underlying complex objects. For examples: len(), reversed(), enumerate() functions.

In Python, not all built-in functions internally call a method on a class, but most of them do, especially when dealing with objects and special methods.

Typically, built-in functions like len(), str(), int(), iter(), etc., internally call special methods of the object (i.e., methods that start and end with double underscores).

• len(my_obj) internally calls myobj.__len__().
• str(obj) internally calls obj._str__().
• reversed() internally calls the __**reversed__**() method.

Because of this, we can override these object methods in the custom class to control what the built-in function will returns for the instance of your class.

Class List:

# Intersection in OOP and function

class List:
    def __init__(self, args):
        # args: allow functions to accept an arbitrary number of arguments.
        # can be a list, string, integer, etc.
        self._list = args

    # Override the __len__() method
    def __len__(self):
        return 3

def main():
    my_list = List([1, 2, 3, 4, 5])
    print("Length of List:", len(my_list))

main()

Output:

Length of List: 3

Class String: