Use if x is not None or if not x is None

Should I use if x is not None or if not x is None?

Google's style guide and PEP-8 both use if x is not None, so is there some slight performance difference between them?

It was found through testing that there is no performance difference because 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

But in terms of usage style, try to avoid not x is y. Although the compiler always treats this as not (x is y), readers may misunderstand the construction as (not x) is y. So if x is not y there is no such ambiguity.

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