<s>[INST] Python `if x is not None` or `if not x is None`? I've always thought of the if not x is None version to be more clear, but Google's style guide implies (based on this excerpt) that they use if x is not None. Is there any minor performance difference (I'm assuming not), and is there any case where one really doesn't fit (making the other a clear winner for my convention)?*
*I'm referring to any singleton, rather than just None.

...to compare singletons like
  None. Use is  or is not.

 [/INST] There's no performance difference, as they compile to the same bytecode:
Python 2.6.2 (r262:71600, Apr 15 2009, 07:20:39)
>>> import dis
>>> def f(x):
...    return x is not None
...
>>> dis.dis(f)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (None)
              6 COMPARE_OP               9 (is not)
              9 RETURN_VALUE
>>> def g(x):
...   return not x is None
...
>>> dis.dis(g)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (None)
              6 COMPARE_OP               9 (is not)
              9 RETURN_VALUE

Stylistically, I try to avoid not x is y.  Although the compiler will always treat it as not (x is y), a human reader might misunderstand the construct as (not x) is y.  If I write x is not y then there is no ambiguity.
 </s>