Is it reasonable to use None as a dictionary key in Python?

Any hashable value is a valid Python Dictionary Key. For this reason, None is a perfectly valid candidate. There’s no confusion when looking for non-existent keys – the presence of None as a key would not affect the ability to check for whether another key was present. Ex:

>>> d = {1: 'a', 2: 'b', None: 'c'}
>>> 1 in d
True
>>> 5 in d
False
>>> None in d
True

There’s no conflict, and you can test for it just like normal. It shouldn’t cause you a problem. The standard 1-to-1 Key-Value association still exists, so you can’t have multiple things in the None key, but using None as a key shouldn’t pose a problem by itself.