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