Python “x 不在”与“x 不在”

"x not in" vs. "not x in"

提问by avacariu

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

我注意到这两者的工作方式相同：

if x not in listand if not x in list.

if x not in list和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?

在某些情况下，两者之间是否存在某种差异？是否有理由同时拥有两者，或者仅仅是因为某些人写一个或另一个更自然？

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

我更可能在其他人的代码中看到哪一个？

采纳答案by Alex Martelli

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

这两种形式生成相同的字节码，您可以清楚地验证：

>>> 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.

所以很明显它们在语义上是相同的。

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

就风格而言，PEP 8没有提到这个问题。

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

就我个人而言，我非常喜欢这种if x not in y形式——它立即表明它not in是一个单一的运算符，并且“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if (not x) in y，读起来不像英语，完全没有补偿优势。

回答by Ned Batchelder

It just personal preference. You could also compare if x != 3and if not x == 3. There's no difference between the two expressions you've shown.

这只是个人喜好。您还可以比较if x != 3和if not x == 3。您展示的两个表达式之间没有区别。

回答by habnabit

not x in Lisn't explicitly disallowed because that would be silly. x not in Lis explicitly allowed (though it compiles to the same bytecode) because it's more readable.

not x in L没有明确禁止，因为那很愚蠢。x not in L明确允许（尽管它编译为相同的字节码），因为它更具可读性。

x not in Lis what everyone uses, though.

x not in L但是，这是每个人都使用的。

回答by Mark Byers

When you write a not in bit is using the not inoperator, whereas not a in buses the inoperator and then negates the result. But the not inoperator is defined to return the same as not a in bso they do exactly the same thing. From the documentation:

当您编写a not in b它时使用not in运算符，而not a in b使用in运算符然后否定结果。但是not in运算符被定义为返回相同的内容，not a in b因此它们执行完全相同的操作。从文档：

The operators inand not intest for collection membership. x in sevaluates to true if xis a member of the collection s, and false otherwise. x not in sreturns the negation of x in s.

运算符in和not in集合成员资格的测试。x in s如果x是集合的成员，则评估为真，否则为s假。x not in s返回 的否定x in s。

Similarly there is a is not bversus not a is b.

同样有a is not bvs not a is b。

The extra syntax was added because it makes it easier for a human to read it naturally.

添加了额外的语法是因为它使人们更容易自然地阅读它。

回答by killown

>>> dis.dis(lambda: a not in b)
1           0 LOAD_GLOBAL              0 (a)
          3 LOAD_GLOBAL              1 (b)
          6 COMPARE_OP               7 (not in)
          9 RETURN_VALUE      

>>> dis.dis(lambda: not a in b)
1           0 LOAD_GLOBAL              0 (a)
          3 LOAD_GLOBAL              1 (b)
          6 COMPARE_OP               7 (not in)
          9 RETURN_VALUE  

when you do "not a in b" it will need be converted for (not in)

当你做“not a in b”时，它需要被转换为（not in）

so, the right way is "a not in b".

所以，正确的方法是“a not in b”。