Why ‘if x is not None’ Beats ‘if not x’ in Python: Avoid Hidden Bugs

In Python, checking whether a variable is None can be written in three common ways: if x is None , if not x , and if not x is None (interpreted as if not (x is None) ).

First form: if x is None

Second form: if not x

Third form: if not x is None

Although these appear equivalent, they behave differently because Python treats None , False , empty string "" , zero 0 , empty list [] , empty dict {} , and empty tuple () as falsy values. Therefore if not x will also trigger for any of those falsy objects, which can lead to bugs when you intend to distinguish None from an empty container.

For example, when using a list, if not x cannot differentiate between x == [] and x is None . This may cause logic errors if you only wanted to test for None .

Consequently, Python developers who prefer if not x must be aware that it treats all falsy values the same and ensure that such values do not affect their logic.

Both if x is not None and if not x is None are clearer than if not x , but the former is unambiguous and recommended by Google’s style guide.

Conclusion: Use if x is not None for the clearest and safest None check. The if not x pattern should only be used when you are certain that x will never be any other falsy value.