String comparison in Python: is vs. ==
comparison, equality, python, string
Solution
For all built-in Python objects (like strings, lists, dicts, functions, etc.), if x is y, then x==y is also True.
Not always. NaN is a counterexample. But usually, identity (`is`) implies equality (`==`). The converse is not true: Two distinct objects can have the same value.
Also, is it generally considered better to just use '==' by default, even when comparing int or Boolean values?
You use `==` when comparing values and `is` when comparing identities.
When comparing ints (or immutable types in general), you pretty much always want the former. There's an optimization that allows small integers to be compared with `is`, but don't rely on it.
For boolean values, you shouldn't be doing comparisons at all. Instead of:
if x == True:
# do something
write:
if x:
# do something
For comparing against `None`, `is None` is preferred over `== None`.
I've always liked to use 'is' because I find it more aesthetically pleasing and pythonic (which is how I fell into this trap...), but I wonder if it's intended to just be reserved for when you care about finding two objects with the same id.
Yes, that's exactly what it's for.
Problem
I noticed a Python script I was writing was acting squirrelly, and traced it to an infinite loop, where the loop condition was `while line is not ''`. Running through it in the debugger, it turned out that line was in fact `''`. When I changed it to `!=''` rather than `is not ''`, it worked fine. Also, is it generally considered better to just use '==' by default, even when comparing int or Boolean values? I've always liked to use 'is' because I find it more aesthetically pleasing and pythonic (which is how I fell into this trap...), but I wonder if it's intended to just be reserved for when you care about finding two objects with the same id.