Python checking if exactly one object is not None

Question

I have two python objects a and b.

What is the best/most-efficient/most-pythonic way to check if exactly one of these objects is None?

You seem to be asking 3 different questions. Do you want the best, the most efficient, or the most Pythonic? The first and third are opinions, so off-topic. The 2nd is probably not important: [Premature optimization is the root of all evil](http://c2.com/cgi/wiki?PrematureOptimization) — Barmar, May 25 '18 at 17:32
`sum(x is None for x in (a,b)) == 1` that sounds overkill, though :) — Jean-François Fabre, May 25 '18 at 17:34
@Jean-FrançoisFabre that trades on the booleans in Python being numbers, and I know they are, but it still bugs me that we leverage it. It makes me feel dirty in a type-violation sort of way. — pcurry, May 25 '18 at 17:37
booleans are integers. `True == 1`, `False == 0`. that's written in stone — Jean-François Fabre, May 25 '18 at 17:38

score 10 · Answer 1 · answered May 25 '18 at 17:34

10

Use Python's ^ (XOR) operator.

(a is None) ^ (b is None)

answered May 25 '18 at 17:34

Bill the Lizard

1

I am torn as to whether or not it is preferable to use `!=`. Yes, logical and bitwise XOR are equivalent for booleans, but still... – juanpa.arrivillaga May 25 '18 at 17:35
@juanpa.arrivillaga I'm torn too. When I think "exactly one of two booleans is True" I automatically think XOR, but `!=` fits just as well. – Bill the Lizard May 25 '18 at 17:39
This is why I referenced the older question here: https://stackoverflow.com/questions/432842/how-do-you-get-the-logical-xor-of-two-variables-in-python – pcurry May 25 '18 at 17:40
Yes, for me the problem is the distinction between bitwise and logical operators. Unfortunately, python doesn't have a logical exclusive-or operator. When I glance at the code and see the bitwise `^`, I think "there's some bitwise operation going on here", but on the other hand, `!=` isn't immediatebly "XOR" in my head. – juanpa.arrivillaga May 25 '18 at 17:41
I did a quick benchmark and ^ seems to be marginally more efficient. – Olivier Melançon May 25 '18 at 17:44

score 5 · Answer 2 · answered May 25 '18 at 17:35

What you effectively want is the XOR or Exclusive OR function on whether those two objects are None. The following should work for you:

(a is None) ^ (b is None)

A more exhaustive answer on how to get XOR on objects in Python can be found here:

2 Answers2