Python `if x is not None` or `if not x is None`? [closed]

There's no performance difference, as they compile to the same bytecode:

>>> import dis>>> dis.dis("not x is None")  1           0 LOAD_NAME                0 (x)              2 LOAD_CONST               0 (None)              4 COMPARE_OP               9 (is not)              6 RETURN_VALUE>>> dis.dis("x is not None")  1           0 LOAD_NAME                0 (x)              2 LOAD_CONST               0 (None)              4 COMPARE_OP               9 (is not)              6 RETURN_VALUE

Stylistically, I try to avoid not x is y, a human reader might misunderstand it as (not x) is y. If I write x is not y then there is no ambiguity.

Both Google and Python's style guide is the best practice:

if x is not None:    # Do something about x

Using not x can cause unwanted results.

See below:

>>> x = 1>>> not xFalse>>> x = [1]>>> not xFalse>>> x = 0>>> not xTrue>>> x = [0]         # You don't want to fall in this one.>>> not xFalse

You may be interested to see what literals are evaluated to True or False in Python:

• Truth Value Testing

Edit for comment below:

I just did some more testing. not x is None doesn't negate x first and then compared to None. In fact, it seems the is operator has a higher precedence when used that way:

>>> x[0]>>> not x is NoneTrue>>> not (x is None)True>>> (not x) is NoneFalse

Therefore, not x is None is just, in my honest opinion, best avoided.

More edit:

I just did more testing and can confirm that bukzor's comment is correct. (At least, I wasn't able to prove it otherwise.)

This means if x is not None has the exact result as if not x is None. I stand corrected. Thanks bukzor.

However, my answer still stands: Use the conventional if x is not None. :]

Code should be written to be understandable to the programmer first, and the compiler or interpreter second. The "is not" construct resembles English more closely than "not is".