Python if x is not None or if not x is None

Question

I ve always thought of the if not x is None version to be more clear  but Google s style guide and PEP-8 both use if x is not None  Is there any minor performance difference  I m assuming not   and is there any case where one really doesn t fit  making the other a clear winner for my convention      I m referring to any singleton  rather than just None         to compare singletons like   None  Use is  or is not

User · Answer

Both Google and Python s style guide is the best practice   if x is not None        Do something about x   Using not x can cause unwanted results    See below    gt  gt  gt  x   1  gt  gt  gt  not x False  gt  gt  gt  x    1   gt  gt  gt  not x False  gt  gt  gt  x   0  gt  gt  gt  not x True  gt  gt  gt  x    0            You don t want to fall in this one   gt  gt  gt  not x False   You may be interested to see what literals are evaluated to True or False in Python    Truth Value Testing     Edit for comment below    I just did some more testing  not x is None doesn t negate x first and then compared to None  In fact  it seems the is operator has a higher precedence when used that way    gt  gt  gt  x  0   gt  gt  gt  not x is None True  gt  gt  gt  not  x is None  True  gt  gt  gt   not x  is None False   Therefore  not x is None is just  in my honest opinion  best avoided     More edit   I just did more testing and can confirm that bukzor s comment is correct   At least  I wasn t able to prove it otherwise    This means if x is not None has the exact result as if not x is None  I stand corrected  Thanks bukzor   However  my answer still stands  Use the conventional if x is not None

User · Answer

The answer is simpler than people are making it   There s no technical advantage either way  and  x is not y  is what everybody else uses  which makes it the clear winner   It doesn t matter that it  looks more like English  or not  everyone uses it  which means every user of Python--even Chinese users  whose language Python looks nothing like--will understand it at a glance  where the slightly less common syntax will take a couple extra brain cycles to parse   Don t be different just for the sake of being different  at least in this field

User · Answer

Code should be written to be understandable to the programmer first  and the compiler or interpreter second  The  is not  construct resembles English more closely than  not is

User · Answer

There s no performance difference  as they compile to the same bytecode   gt  gt  gt  import dis  gt  gt  gt  dis dis  quot not x is None quot     1           0 LOAD NAME                0  x                2 LOAD CONST               0  None                4 COMPARE OP               9  is not                6 RETURN VALUE  gt  gt  gt  dis dis  quot x is not None quot     1           0 LOAD NAME                0  x                2 LOAD CONST               0  None                4 COMPARE OP               9  is not                6 RETURN VALUE  Stylistically  I try to avoid not x is y  a human reader might misunderstand it as  not x  is y  If I write x is not y then there is no ambiguity

User · Answer

Personally  I use   if not  x is None     which is understood immediately without ambiguity by every programmer  even those not expert in the Python syntax

User · Answer

Python if x is not None or if not x is None    TLDR  The bytecode compiler parses them both to x is not None - so for readability s sake  use if x is not None   Readability  We use Python because we value things like human readability  useability  and correctness of various paradigms of programming over performance   Python optimizes for readability  especially in this context   Parsing and Compiling the Bytecode  The not binds more weakly than is  so there is no logical difference here  See the documentation      The operators is and is not test for object identity  x is y is true   if and only if x and y are the same object  x is not y yields the   inverse truth value    The is not is specifically provided for in the Python grammar as a readability improvement for the language    comp op    lt     gt          gt      lt      lt  gt         in   not   in   is   is   not    And so it is a unitary element of the grammar as well    Of course  it is not parsed the same    gt  gt  gt  import ast  gt  gt  gt  ast dump ast parse  x is not None   body 0  value   Compare left Name id  x   ctx Load     ops  IsNot     comparators  Name id  None   ctx Load        gt  gt  gt  ast dump ast parse  not x is None   body 0  value   UnaryOp op Not    operand Compare left Name id  x   ctx Load     ops  Is     comparators  Name id  None   ctx Load          But then the byte compiler will actually translate the not     is to is not    gt  gt  gt  import dis  gt  gt  gt  dis dis lambda x  y  x is not y    1           0 LOAD FAST                0  x                3 LOAD FAST                1  y                6 COMPARE OP               9  is not                9 RETURN VALUE  gt  gt  gt  dis dis lambda x  y  not x is y    1           0 LOAD FAST                0  x                3 LOAD FAST                1  y                6 COMPARE OP               9  is not                9 RETURN VALUE   So for the sake of readability and using the language as it was intended  please use is not   To not use it is not wise

User · Answer

if not x is None is more similar to other programming languages  but if x is not None definitely sounds more clear  and is more grammatically correct in English  to me   That said it seems like it s more of a preference thing to me

User · Answer

I would prefer the more readable form x is not y than I would think how to eventually write the code handling precedence of the operators in order to produce much more readable code

User · Answer

The is not operator is preferred over negating the result of is for stylistic reasons   if x is not None   reads just like English  but  if not x is None   requires understanding of the operator precedence and does not read like english   If there is a performance difference my money is on is not  but this almost certainly isn t the motivation for the decision to prefer that technique  It would obviously be implementation-dependent  Since is isn t overridable  it should be easy to optimise out any distinction anyhow

[python] Python `if x is not None` or `if not x is None`?

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