Python for and if在一行上 [英] Python for and if on one line

问题描述

我在使用python时遇到问题.

I have a issue with python.

我列出一个简单的列表:

I make a simple list:

>>> my_list = ["one","two","three"]

我想创建一个单行代码"来查找字符串.

I want create a "single line code" for find a string.

例如，我有以下代码:

>>> [(i) for i in my_list if i=="two"]
['two']

但是当我看到变量是错误的时(我找到了列表的最后一个值):

But when I watch the variable is wrong (I find the last value of my list):

>>> print i
three

为什么我的变量包含最后一个元素而不包含我要查找的元素?

Why does my variable contain the last element and not the element that I want to find?

推荐答案

您正在通过使用列表理解来生成过滤列表. i仍绑定到该列表的每个元素，并且最后一个元素仍然是'three'，即使随后已从要生成的列表中将其滤除.

You are producing a filtered list by using a list comprehension. i is still being bound to each and every element of that list, and the last element is still 'three', even if it was subsequently filtered out from the list being produced.

您不应使用列表推导来挑选一个元素.只需使用for循环，然后使用break结束循环即可:

You should not use a list comprehension to pick out one element. Just use a for loop, and break to end it:

for elem in my_list:
    if elem == 'two':
        break

如果您必须使用单行代码(这与Python的哲学相反，在该哲学中，可读性很重要)，请使用

If you must have a one-liner (which would be counter to Python's philosophy, where readability matters), use the next() function and a generator expression:

i = next((elem for elem in my_list if elem == 'two'), None)

，如果没有这样的匹配元素，则会将i设置为None.

which will set i to None if there is no such matching element.

上面的过滤器不是那么有用；您实际上是在测试值'two'是否在列表中.您可以为此使用in

The above is not that useful a filter; your are essentially testing if the value 'two' is in the list. You can use in for that:

elem = 'two' if 'two' in my_list else None

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