How to assert that zero or only one of N given arguments is passed

Question

I have a definition like this

def bar(self, foo=None, bar=None, baz=None):
    pass

I want to make sure a maximum of one of foo, bar, baz is passed. I can do

if foo and bar:
    raise Ex()

if foo and baz:
    raise Ex()
....

But there got be something simpler.

Answer 1

How about:

 initialisers = [foo, bar, baz]
 if initialisers.count(None) < len(initialisers) - 1:
     raise Ex()

It simply counts how many None are present. If they're all None or only one isn't then fine, otherwise it raises the exception.