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,

#!python3    class Foo:   num = 1   pass  class Bar:   num = 2   pass  f1,f2= Foo(), Foo()  f2.__class__ = Bar print( type(f1), type(f2), f1.num, f2.num ) 

The result shows:

<class '__main__.Foo'> <class '__main__.Bar'> 1 2

I think I changed the type of f2.
What's wrong, What am I missing?

Answer 1

The footnotes one that page says:

[1] It is possible in some cases to change an object’s type, under certain controlled conditions. It generally isn’t a good idea though, since it can lead to some very strange behaviour if it is handled incorrectly.

If you try to change the __class__ of f2 to list:

f2.__class__ = list 

A TypeError raised:

TypeError: __class__ assignment: only for heap types 

Answer 2

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.