“x不在"与“不是 x in" [英] "x not in" vs. "not x in"

问题描述

我注意到这两者的工作方式相同:

if x not in list and if not x in list.

在某些情况下，两者之间是否存在某种差异?两者兼而有之，还是只是因为有些人写一个或另一个更自然?

我更有可能在其他人的代码中看到哪个?

解决方案

这两种形式产生了相同的字节码，你可以清楚地验证:

<预><代码>>>>导入文件>>>dis.dis(compile('if x not in d: pass', '', 'exec'))1 0 LOAD_NAME 0 (x)3 LOAD_NAME 1 (d)6 COMPARE_OP 7(不在)9 JUMP_IF_FALSE 4(到 16)12 POP_TOP13 JUMP_FORWARD 1(到 17)>>16 POP_TOP>>17 LOAD_CONST 0 (无)20 RETURN_VALUE>>>dis.dis(compile('if not x in d: pass', '', 'exec'))1 0 LOAD_NAME 0 (x)3 LOAD_NAME 1 (d)6 COMPARE_OP 7(不在)9 JUMP_IF_FALSE 4(到 16)12 POP_TOP13 JUMP_FORWARD 1(到 17)>>16 POP_TOP>>17 LOAD_CONST 0 (无)20 RETURN_VALUE

很明显它们在语义上是相同的.

就风格而言，PEP 8 没有提到这个问题.

就我个人而言，我非常喜欢 if x not in y 形式——这立即表明 not in 是一个单一的操作符，and强>读起来像英语".if not x in y 可能会误导一些读者认为它的意思是if (not x) in y，读起来不像英语，完全没有补偿优势.

I've noticed that both of these work the same:

if x not in list and if not x in list.

Is there some sort of difference between the two in certain cases? Is there a reason for having both, or is it just because it's more natural for some people to write one or the other?

Which one am I more likely to see in other people's code?

解决方案

The two forms make identical bytecode, as you can clearly verify:

>>> import dis
>>> dis.dis(compile('if x not in d: pass', '', 'exec'))
  1           0 LOAD_NAME                0 (x)
              3 LOAD_NAME                1 (d)
              6 COMPARE_OP               7 (not in)
              9 JUMP_IF_FALSE            4 (to 16)
             12 POP_TOP             
             13 JUMP_FORWARD             1 (to 17)
        >>   16 POP_TOP             
        >>   17 LOAD_CONST               0 (None)
             20 RETURN_VALUE        
>>> dis.dis(compile('if not x in d: pass', '', 'exec'))
  1           0 LOAD_NAME                0 (x)
              3 LOAD_NAME                1 (d)
              6 COMPARE_OP               7 (not in)
              9 JUMP_IF_FALSE            4 (to 16)
             12 POP_TOP             
             13 JUMP_FORWARD             1 (to 17)
        >>   16 POP_TOP             
        >>   17 LOAD_CONST               0 (None)
             20 RETURN_VALUE        

so obviously they're semantically identical.

As a matter of style, PEP 8 does not mention the issue.

Personally, I strongly prefer the if x not in y form -- that makes it immediately clear that not in is a single operator, and "reads like English". if not x in y may mislead some readers into thinking it means if (not x) in y, reads a bit less like English, and has absolutely no compensating advantages.

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