Your note is very important, I think it would be good to give it more emphasis.

What works flawlessly, however, is:

  In [1]: a = None
  In [2]: b = 0
  In [3]: (a is None) ^ (b is None)
  Out[3]: True

Alternatively, as suggested in another comment, you can use inequality as a replacement for XOR:

  In [4]: (a is None) != (b is None)
  Out[4]: True

Another error prone pattern is the following:

  In [5]: a or b
  Out[5]: 0

Which can behave differently depending on the order of elements/values:

  In [6]: b or a
  Out[6]: None # should be 0

Since b is zero, it doesn't count. Less error prone, but also more verbose is:

  In [10]: a if a is not None else b
  Out[10]: 0