如果不是Python == vs if！= [英] Python if not == vs if !=

问题描述

这两行代码有什么区别：

What is the difference between these two lines of code:

if not x == 'val':

和

if x != 'val':

一个比另一个更有效吗？

Is one more efficient than the other?

如果x =='val'，使用

Would it be better to use

if x == 'val':
    pass
else:

推荐答案

使用 dis 查看为两个版本生成的字节码：

Using dis to look at the bytecode generated for the two versions:

not ==

not ==

  4           0 LOAD_FAST                0 (foo)
              3 LOAD_FAST                1 (bar)
              6 COMPARE_OP               2 (==)
              9 UNARY_NOT           
             10 RETURN_VALUE   

！=

!=

  4           0 LOAD_FAST                0 (foo)
              3 LOAD_FAST                1 (bar)
              6 COMPARE_OP               3 (!=)
              9 RETURN_VALUE   

后者的操作较少，因此可能稍微提高效率。

The latter has fewer operations, and is therefore likely to be slightly more efficient.

有人指出在通讯中（谢谢， @Quincunx ）在哪里你有如果foo！= bar 与如果不是foo == bar 操作次数完全相同，只是 COMPARE_OP 更改而 POP_JUMP_IF_TRUE 切换为 POP_JUMP_IF_FALSE ：

It was pointed out in the commments (thanks, @Quincunx) that where you have if foo != bar vs. if not foo == bar the number of operations is exactly the same, it's just that the COMPARE_OP changes and POP_JUMP_IF_TRUE switches to POP_JUMP_IF_FALSE:

not == ：

not ==:

  2           0 LOAD_FAST                0 (foo)
              3 LOAD_FAST                1 (bar)
              6 COMPARE_OP               2 (==)
              9 POP_JUMP_IF_TRUE        16

！=

!=

  2           0 LOAD_FAST                0 (foo)
              3 LOAD_FAST                1 (bar)
              6 COMPARE_OP               3 (!=)
              9 POP_JUMP_IF_FALSE       16

在这种情况下，除非每次比较所需的工作量有所不同，否则您根本不会看到任何性能差异。

In this case, unless there was a difference in the amount of work required for each comparison, it's unlikely you'd see any performance difference at all.

但请注意，两个版本在逻辑上并不总是相同，因为它取决于 __ eq __ 和 __ ne __ 。根据数据模型文档：

However, note that the two versions won't always be logically identical, as it will depend on the implementations of __eq__ and __ne__ for the objects in question. Per the data model documentation:

比较运算符之间没有隐含的关系。 x == y 的
真值并不意味着 x！= y 为假。

例如：

>>> class Dummy(object):
    def __eq__(self, other):
        return True
    def __ne__(self, other):
        return True


>>> not Dummy() == Dummy()
False
>>> Dummy() != Dummy()
True

---

最后，也许最重要的是：一般来说，两个在逻辑上相同的地方， x！= y 更多可读而不是x == y 。

---

Finally, and perhaps most importantly: in general, where the two are logically identical, x != y is much more readable than not x == y.

这篇关于如果不是Python == vs if！=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！