Safely bind method from one class to another class in Python [duplicate]

Question

I know I can attach a function to a class and make it a method:

 >>> def is_not_bound(inst, name):
...     print("Hello %s" % name)
... 
>>> 
>>> class NoMethods:
...     pass
... 
>>> 
>>> setattr(NoMethods, 'bound', is_not_bound)
>>> 
>>> NoMethods().bound("oz") # prints: Hello oz
Hello oz

To my surprise this works also with a bound method from one class to another:

>>> class Foo:
...     def foo(self, name):
...         print("Hello %s" % name)
... 
>>> class B:
...     pass
... 
>>> setattr(B, 'bound_to_b', getattr(Foo, 'foo'))
>>> B().bound_to_b("confused?")
Hello confused?
>>> 

Can I safely use this? Is there something I am overseeing?

update

One caveat that I found already:

>>> B.bound_to_b
<function Foo.foo at 0x7fc997e8b730>

Even though I called the method from B, It seems bound to Foo.

And even more surprising!:

>>> def new_method(self, addto):
...     return self.num + addto
... 
>>> setattr(B, 'add', new_method)
>>> b=B()
>>> b.num = 2
>>> b.add(2)
4

Answer 1

Apparently, this is an intended behavior (which is cool!). But also apparently, this behavior is not very familiar.

If you knew Python 2 for a long time, you would might not be aware of the fact that Python 3 has no methods (as commented above).

So in Python 3:

>>> class Foo:
...     def foo(self, name):
...         print("Hello %s" % name)
... 
>>> Foo.foo
<function Foo.foo at 0x7f729a406730>
>>> def foo():
...     pass
... 
>>> foo
<function foo at 0x7f729b83ff28>
>>> 
>>> Foo.foo
<function Foo.foo at 0x7f729a406730>

There is no distinction! In Python 2 however:

Python 2.7.14 (default, Feb  2 2018, 02:17:12) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo:
...     def foo(self, name):
...         print("Hello %s" % name)
... 
>>> Foo.foo
<unbound method Foo.foo>
>>>