not None test in Python

Question

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

User · Answer

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.

User · Answer

if val is not None              is the Pythonic idiom for testing that a variable is not set to None  This idiom has particular uses in the case of declaring keyword functions with default parameters  is tests identity in Python  Because there is one and only one instance of None present in a running Python script program  is is the optimal test for this  As Johnsyweb points out  this is discussed in PEP 8 under  Programming Recommendations    As for why this is preferred to   if not  val is None               this is simply part of the Zen of Python   Readability counts   Good Python is often close to good pseudocode

User · Answer

Either of the latter two  since val could potentially be of a type that defines   eq     to return true when passed None

User · Answer

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

[python] not None test in Python

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