not None test in Python

python nonetype
prosseek picture prosseek · Oct 19, 2010 · Viewed 854.6k times · Source

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?

Answer

gotgenes picture gotgenes · Oct 19, 2010 
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.

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