Using superclass methods as instance methods

Question

I am currently reading through Luciano Ramalho's excellent book Fluent Python. In a chapter about interfaces and inheritance we build a subclass of a list (see github for the original code) and I am confused about the way we define one of the instance methods. For a simlified example my confusion is caused by a situation as follows:

class ListWithLoadMethod(list):
    load = list.extend

which generates a new subclass of list which has an alias for the extend method as load. We can test the class by writing

loaded_list = ListWithLoadMethod(range(4))
print(loaded_list)
loaded_list.extend(range(3))
print(loaded_list)
loaded_list.load(range(3))
print(loaded_list)

which produces, as expected:

[0, 1, 2, 3]
[0, 1, 2, 3, 0, 1, 2]
[0, 1, 2, 3, 0, 1, 2, 0, 1, 2]

My confusion arises from on the difference of class methods and static methods. When the new instance of ListWithLoadedMethod is created, it is a subclass of list, but when we initialize the instance we point load to list.extend; how does Python know that by list.extend we do not mean that load should point to a class method of the list class but to actually (apparently?) inherit the instance method of the superclass list?

Ashwani Tanwar · Accepted Answer · 2021-06-02 05:31:19Z

7

It's actually not inheriting from superclass (list), but creating reference to list.extend method

When you inspect their identity, you will see that they are same objects in memory.

id(list.extend)
Out: 3102044666032

id(ListWithLoadMethod.load)
Out: 3102044666032

edited Jun 2, 2021 at 5:31

Ashwani Tanwar

1712 silver badges11 bronze badges

answered Jun 2, 2021 at 5:22

Kholdarbekov

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3 Comments

Rami Luisto Over a year ago

Thanks! I have to remember the id function from now on ^^ So, to extend my question if you don't mind: what happens when an instance of ListWithLoadMethod calls list.extend? Does list.extend(iterable) actually look like list.extend(self, iterable) when in use or what?

Kholdarbekov Over a year ago

When you call list.extend on instance of ListWithLoadMethod below happens: 1) l_obj = ListWithLoadMethod() 2) ListWithLoadMethod.extend(l_obj, iterable). So it works similar other class methods

Rami Luisto Over a year ago

Thanks again! You've been a great help.

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