Integer object identity gotcha

Given Python's distinction between object identity and equality, it might come as a surprise to find that:

>>> a = 256
>>> b = 256
>>> a is b
True

This happens because Python keeps a cache of commonly-used, small integer objects (on my system, -5 – 256). To improve performance, the assignment a = 256 attaches the variable name a to the existing integer object without having to allocate new memory for it. Since the same thing happens with b, the two variables in this case do, in fact, point to the same object. By contrast,

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