Python 2 draws a distinction between bound and unbound methods, whereas in Python 3 this distinction is gone: unbound methods have been removed from the language. To bind a method to a class compatibly across Python 3 and Python 2, you can use the :func:`bind_method` helper function:

from future.utils import bind_method

class Greeter(object):
    pass

def greet(self, message):
    print(message)

bind_method(Greeter, 'greet', greet)

g = Greeter()
g.greet('Hi!')

On Python 3, calling bind_method(cls, name, func) is equivalent to calling setattr(cls, name, func). On Python 2 it is equivalent to:

import types
setattr(cls, name, types.MethodType(func, None, cls))