changing the class of a python object (casting)

Question

On this python doc page it says:

Like its identity, an object’s type is also unchangeable.

And I try this script,

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Answer 1

When and how to do this

Changing the type ("casting") makes sense if you want to add functionality to an object created by some code you cannot change.

Assume some statement obj = some_call_to_a_library() gives you an object of class A. You want it to have additional functionality, say, mymethod(). Then you could introduce a subclass MyA like this (Python 3 style):

class MyA(A):
    @classmethod
    def cast(cls, some_a: A):
        """Cast an A into a MyA."""
        assert isinstance(some_a, A)
        some_a.__class__ = cls  # now mymethod() is available
        assert isinstance(some_a, MyA)
        return some_a

    def mymethod(self):
        ...

and then write obj = MyA.cast(some_call_to_a_library()). If MyA relies on additional attributes, cast (which is a factory method) should create them.

I just did something like this when I needed a version of requests.Response that could persist and retrieve responses to/from a file.