How does the Python runtime know which object a method is bound to?

Question

When defining methods in a class we do have an argument for the object on which the method is invoked i.e self.

Say I have a class Foo.

class Foo:
   def m(self):
      pass

now when we have an object of Foo, say foo.

and we do something like this,

foo_m = foo.m

and then try calling foo_m with the same parameters the way would have done with foo.m. It just works.

So How does foo_m knows what should be the value of the self argument when it is not being called in foo.m() fashion ?

on printing the foo.m, the REPL shows that its a bound method to that particular object.

Is it some other function object that keeps track of the self argument and the method to be called ?

Answer 1

A bound method is just a Python object that stores self in one of its slots. Take a look at methodobject.c for the C implementation. In Python the slot is accessible via the __self__ attribute:

>>> a = [1, 2, 3].append
>>> a
<built-in method append of list object at 0x103b39588>
>>> a.__self__
[1, 2, 3]