Question

Given two non-related classes A and B, how to call A.method with an object of B as self?

class A:
    def __init__(self, x):
        self.x = x

    def print_x(self):
        print self.x

class B:
    def __init__(self, x):
        self.x = x

a = A('spam')
b = B('eggs')

a.print_x() #<-- spam
<magic>(A.print_x, b) #<-- 'eggs'

Answer 1

In Python 3.x you can simply do what you want:

A.print_x(b) #<-- 'eggs'

If you only have an instance of ‘A’, then get the class first:

a.__class__.print_x(b) #<-- 'eggs'

In Python 2.x (which the OP uses) this doesn’t work, as noted by the OP and explained by Amber in the comments:

This is a difference between Python 2.x and Python 3.x – methods in
3.x don’t enforce being passed the same class.

More details (OP edit)

In python 2, A.print_x returns an “unbound method”, which cannot be directly applied to other classes’ objects:

When an unbound user-defined method object is called, the underlying function (im_func) is called, with the restriction that the first argument must be an instance of the proper class (im_class) or of a derived class thereof. >> http://docs.python.org/reference/datamodel.html

To work around this restriction, we first have to obtain a “raw” function from a method, via im_func or __func__ (2.6+), which then can be called passing an object. This works on both classes and instances:

# python 2.5-
A.print_x.im_func(b)    
a.print_x.im_func(b)

# python 2.6+
A.print_x.__func__(b)   
a.print_x.__func__(b)

In python 3 there’s no such thing anymore as unbound method.

Unbound methods are gone for good. ClassObject.method returns an
ordinary function object, instance.method still returns a bound
method object. >> http://www.python.org/getit/releases/3.0/NEWS.txt

Hence, in python 3, A.print_x is just a function, and can be called right away and a.print_x still has to be unbounded:

# python 3.0+
A.print_x(b)
a.print_x.__func__(b)