foo.py

class Foo(object):
    def __init__(self):
        self.x = 0

    def __repr__(self):
        return "<Foo x:%d>" % self.x

    def add_x(self, x):
        self.x += x

def add_x(foo, x):
    foo.x += x

f = Foo()
print repr(f)
f.add_x(1)
print repr(f)
# I wouldn't, in practice, EVER do this in Python, even though it works:
add_x(f, 1)
print repr(f)

outputs:

<Foo x:0>
<Foo x:1>
<Foo x:2>