What is the pythonic way to check that both values are None or not None together

I a have a function with 2 variables set be default to None

def foo(x, y=None, z=None):
    ...

I want to make sure that if they are passed they both need to be not None.

I did the following:

if y is not None:
    assert z is not None

But I wonder if there is a more elegant way of doing this check.

If only 1 of them is None I want to assert while if both of them are it’s ok.

>Solution :

You can do something like this

if (y is None) == (z is None):
   # both are None or both are not None
else:
  # one of them is None