# An empty class
class EmptyClass:
    pass

# A global function
def fun(self,*arg):
    print(*arg)
    print(self)

# create an instance before poking    
old_instance = EmptyClass()

# poke fun to EmptyClass
EmptyClass.fun = fun

# create an instance after poking    
new_instance = EmptyClass()

# the new_instance has method fun:
new_instance.fun('calling new_instance.fun')
# also the old_instance has method fun:
old_instance.fun('calling old_instance.fun')
"""
calling new_instance.fun
<__main__.EmptyClass object at 0x145b021f3be0>
calling old_instance.fun
<__main__.EmptyClass object at 0x145b021f3c40>
"""