This is the sort of thing that makes me avoid "is" in favor of "==" for most applications.  Understanding when two objects point to the same memory requires a deeper understanding of the underlying code than I usually want to delve into.

Anyway, I find it interesting that for 3.7.3:
>>> a, b = 256, 256
>>> a is b
True

>>> a, b = 257, 257
>>> a is b
False


So 2**8 is a magic number, for whatever reason.  I'll be sticking with "=="...