Python`如果x不是None`或`如果不是x是None？ [英] Python `if x is not None` or `if not x is None`?

问题描述

我一直认为如果不是x是无版本更清楚，但Google的样式指南和 PEP-8 都使用如果x不是无。是否有任何细微的性能差异（我假设不是），是否有一个真的不适合（使另一个明显胜出我的会议）？

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)?*

*我指的是任何单例，而不是无。

*I'm referring to any singleton, rather than just None.

...比较单例如
无。使用是或不是。

...to compare singletons like None. Use is or is not.

推荐答案

没有性能差异， bytecode：

There's no performance difference, as they compile to the same bytecode:

Python 2.6.2 (r262:71600, Apr 15 2009, 07:20:39)
>>> import dis
>>> def f(x):
...    return x is not None
...
>>> dis.dis(f)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (None)
              6 COMPARE_OP               9 (is not)
              9 RETURN_VALUE
>>> def g(x):
...   return not x is None
...
>>> dis.dis(g)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (None)
              6 COMPARE_OP               9 (is not)
              9 RETURN_VALUE

在文体上，我试图避免不是x是y 。虽然编译器总是将它视为 not（x is y），一个人类读者可能会误认为（not x）是y 。如果我写 x不是y ，则没有歧义。

Stylistically, I try to avoid not x is y. Although the compiler will always treat it as not (x is y), a human reader might misunderstand the construct as (not x) is y. If I write x is not y then there is no ambiguity.

这篇关于Python`如果x不是None`或`如果不是x是None？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！