>>> class F(object):
...     def t(self):
...         print 'coucou'
... 
>>> def u():
...     print 'coucou'
... 
>>> def get_instance(func):
...     if not hasattr(func, 'im_self'):
...         return None
...     return func.im_self
... 
>>> get_instance(u)
>>> get_instance(g.t)
<__main__.F object at 0x6c630>