[Python] Why are "self" and "cls" interchangeable as the first argument in class and instance...

I understand from the question "Difference between 'cls' and 'self' in Python classes?" that cls is used in class methods, whereas self is used in instance methods.

However, I used self in class methods and it is still working fine.

For example,

class Test:

@classmethod
def hello(self, name):
print ('hello '+name)

Test.hello('Tom')


What, exactly, does cls or self do in class methods?

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