How to modify a preexisting class in Python in order to add custom methods?

Question

I am new to Python and I am trying to find a way to add new methods to pre-existing classes in Python. For example, to add a .print() method to the list class.

I know I can create a new class that inherits the list class and add this method like so:

    import builtins
    class list(list):
            def print(self):
                    builtins.print(self)

But this doesn't modify the pre-existing 'list' class. I can do assignments like this: trial_list = list([3,4,6]) but not like this: trial_list = [3,4,6].

Also, is there a way to view the actual content of the list class besides dir() and help() ?

Answer 1

The recommended approach to subclass built in container types, is to use the abstract base classes provided in the collections module:

In the case of list , you should use collections.UserList

from collections import UserList

class MySpecialList(UserList):
    def print(self):
        print(self)
        
seq = MySpecialList([1, 2, 3])
print(seq)
seq.print()

You can now create MySpecialList objects the way you do with a plain python list .

Subclassing requirements: Subclasses of UserList are expected to offer a constructor which can be called with either no arguments or one argument. List operations which return a new sequence attempt to create an instance of the actual implementation class. To do so, it assumes that the constructor can be called with a single parameter, which is a sequence object used as a data source.

Of course you can override the methods of list to provide your MySpecialList with the behavior you need.