Collin Funk wrote: > One thing that annoys me personally > is comparing to none using "!=" instead of "is not". This is > recommended against in PEP 8, "Comparisons to singletons like None > should always be done with is or is not, never the equality > operators." [1].
Dima Pasechnik wrote: > The pythonic way is > > mode is not None > > rather than > > mode != None > > (the reason is None is an object) Summarizing the arguments for "is not None": 1- Clarity and readability, best practice, PEP 8. 2- Identity comparison vs. value comparison. 3- None is a singleton. 4- Prevents wrong results if an __eq__ method has been incorrectly coded. Here's my take on it. 1- is clearly subjective. Clarity is context dependent. Etc. 2- Yes, https://docs.python.org/3.12/reference/expressions.html#comparisons clearly explains the difference between == and 'is'. (It's more or less like the difference between EQUAL and EQ in Lisp.) 3- None is a singleton, but it nonetheless supports both == and 'is'. So, this is a weak argument. 4- This is one of the weakest possible arguments. The style warnings about "!= None" in pycodestyle and/or pylint are relativized by this warning in Python itself: >>> '' != None True >>> '' is not None <stdin>:1: SyntaxWarning: "is not" with a literal. Did you mean "!="? True So, if Python itself warns about some code that follows PEP 8... it means that we need to think carefully, rather than blindly apply that PEP 8 rule. I think a better rule, in particular in the context of gnulib-tool.py, is to observe that each variable has a certain set of possible values (even though this set of values is not explicitly stated in the source code), and with this set of possible values comes a comparison operator. So, what I mean is: * If a variable can only contain objects like GLModule instances or None, then 'is' (pointer comparison) is the right choice for this variable. * If a variable can only contain strings or None, then '==' (value comparison) is the right choice for this variable, since comparing strings with 'is' is unreliable: >>> x = "ab" >>> y = "ab" >>> x is y True >>> x = "abc"[:2] >>> y = "abx"[:2] >>> x is y False * If a variable can only contain a list or None, then '==' (value comparison) is the right choice for this variable. >>> ["a"] == ["a"] True >>> ["a"] is ["a"] False So, depending on the variable: mode != None OK modules != None OK module != None not OK, better write: module is not None Bruno
