python中的最佳方法是什么:if语句中的多个OR或IN? [英] What is the best approach in python: multiple OR or IN in if statement?

问题描述

python 中最好的方法是什么:multiple OR 或 IN in if 语句?考虑性能和最佳实践.

What is the best approach in python: multiple OR or IN in if statement? Considering performance and best pratices.

if cond == '1' or cond == '2' or cond == '3' or cond == '4': pass

或

if cond in ['1','2','3','4']: pass

推荐答案

最好的方法是使用set:

if cond in {'1','2','3','4'}:

因为集合中的成员资格测试是 O(1)(恒定成本).

as membership testing in a set is O(1) (constant cost).

另外两种方法的复杂度相同；只是不变成本的差异.列表上的 in 测试和 or 链短路；找到匹配项后立即终止.一个使用字节码跳转序列(如果 True 跳转到末尾)，另一个使用 C 循环和如果值匹配则提前退出.在最坏的情况下，cond 与序列中的元素不匹配，任何一种方法都必须检查所有元素，然后才能返回<代码>假.在这两者中，我会在任何一天选择 in 测试，因为它更具可读性.

The other two approaches are equal in complexity; merely a difference in constant costs. Both the in test on a list and the or chain short-circuit; terminate as soon as a match is found. One uses a sequence of byte-code jumps (jump to the end if True), the other uses a C-loop and an early exit if the value matches. In the worst-case scenario, where cond does not match an element in the sequence either approach has to check all elements before it can return False. Of the two, I'd pick the in test any day because it is far more readable.

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