When to use == and when to use is?

Question

Curiously:

>>> a = 123
>>> b = 123
>>> a is b
True
>>> a = 123.
>>> b = 123.
>>> a is b
False

Seems a is b being more or less defined as id(a) == id(b). It is easy to make bugs this way:

basename, ext = os.path.splitext(fname)
if ext is '.mp3':
    # do something
else:
    # do something else

Some fnames unexpectedly ended up in the else block. The fix is simple, we should use ext == '.mp3' instead, but nonetheless if ext is '.mp3' on the surface seems like a nice pythonic way to write this and it's more readable than the "correct" way.

Since strings are immutable, what are the technical details of why it's wrong? When is an identity check better, and when is an equality check better?

user_282855 · Answer

You should be warned by PyCharm when you use is with a literal with a warning such as SyntaxWarning: "is" with a literal. Did you mean "=="?. So, when comparing with a literal, always use ==. Otherwise, you may prefer using is in order to compare objects through their references.

When to use == and when to use is?

5 Answers