Python if not == vs if != [英] Python if not == vs if !=

问题描述

这两行代码有什么区别:

如果不是 x == 'val':

和

如果 x != 'val':

一种比另一种更有效吗?

使用会更好吗

如果 x == 'val':经过别的:

解决方案

使用 dis 查看两个版本生成的字节码:

not ==

 4 0 LOAD_FAST 0 (foo)3 LOAD_FAST 1(条形)6 COMPARE_OP 2 (==)9 UNARY_NOT10 RETURN_VALUE

!=

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

后者操作较少，因此效率可能稍高一些.

<小时>

在评论中指出(谢谢，<一个 href="https://stackoverflow.com/users/1896169/quincunx">@Quincunx) 那里你有 if foo != bar 与 if not foo== bar 操作次数完全一样，只是COMPARE_OP变了，POP_JUMP_IF_TRUE变成了POP_JUMP_IF_FALSE:

not ==:

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

!=

 2 0 LOAD_FAST 0 (foo)3 LOAD_FAST 1(条形)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不同，否则您根本看不到任何性能差异.

<小时>

但是，请注意，这两个版本在逻辑上并不总是相同，因为这取决于 __eq__ 和 __ne__ 的实现对于有问题的对象.根据数据模型文档:

<块引用>

比较运算符之间没有隐含的关系.这x==y 的真实性并不意味着 x!=y 是错误的.

例如:

<预><代码>>>>类虚拟(对象):def __eq __(自己，其他):返回真def __ne__(自我，其他):返回真>>>not Dummy() == Dummy()错误的>>>假人() !=假人()真的

<小时>

最后，也许是最重要的:一般来说，如果两个 在逻辑上相同，x != y 比 not 更具可读性x == y.

What is the difference between these two lines of code:

if not x == 'val':

and

if x != 'val':

Is one more efficient than the other?

Would it be better to use

if x == 'val':
    pass
else:

解决方案

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

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   

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

---

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 ==:

  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

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.

---

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:

There are no implied relationships among the comparison operators. The truth of x==y does not imply that x!=y is false.

For example:

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


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

---

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

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