Ouch - I didn’t catch that. This will work, on the other hand:
>>> class Dummy(object):
... def __init__(self):
... self.var = 5
...
>>> d = Dummy()
>>> def my_func(cls):
... cls.var = 4
...
>>> my_func(d)
>>> d.var
4
Modifying a class inside a function it was passed to is almost never a good idea; that’s just asking for lots of hard-to-find, hidden bugs.