'not None test in Python [duplicate]

Out of these not None tests.

if val != None:

if not (val is None):

if val is not None:

Which one is preferable, and why?

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Solution 1:[1]

From, Programming Recommendations, PEP 8:

Comparisons to singletons like None should always be done with is or is not, never the equality operators.

Also, beware of writing if x when you really mean if x is not None — e.g. when testing whether a variable or argument that defaults to None was set to some other value. The other value might have a type (such as a container) that could be false in a boolean context!

PEP 8 is essential reading for any Python programmer.

Solution 2:[2]

The best bet with these types of questions is to see exactly what python does. The dis module is incredibly informative:

>>> import dis
>>> dis.dis("val != None")
  1           0 LOAD_NAME                0 (val)
              2 LOAD_CONST               0 (None)
              4 COMPARE_OP               3 (!=)
              6 RETURN_VALUE
>>> dis.dis("not (val is None)")
  1           0 LOAD_NAME                0 (val)
              2 LOAD_CONST               0 (None)
              4 COMPARE_OP               9 (is not)
              6 RETURN_VALUE
>>> dis.dis("val is not None")
  1           0 LOAD_NAME                0 (val)
              2 LOAD_CONST               0 (None)
              4 COMPARE_OP               9 (is not)
              6 RETURN_VALUE

Notice that the last two cases reduce to the same sequence of operations, Python reads not (val is None) and uses the is not operator. The first uses the != operator when comparing with None.

As pointed out by other answers, using != when comparing with None is a bad idea.

Solution 3:[3]

Either of the latter two, since val could potentially be of a type that defines __eq__() to return true when passed None.

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Community
Solution 2 Boris Verkhovskiy
Solution 3 Ignacio Vazquez-Abrams

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