Python for 和 if 在一行
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Python for and if on one line
提问by rostonik
I have a issue with python.
我有 python 的问题。
I make a simple list:
我列一个简单的清单：
>>> my_list = ["one","two","three"]
I want create a "single line code" for find a string.
我想创建一个“单行代码”来查找字符串。
for example, I have this code:
例如，我有这个代码：
>>> [(i) for i in my_list if i=="two"]
['two']
But when I watch the variable is wrong (I find the last value of my list):
但是当我看到变量是错误的（我找到了列表的最后一个值）：
>>> print i
three
Why does my variable contain the last element and not the element that I want to find?
为什么我的变量包含最后一个元素而不是我想要查找的元素？
采纳答案by Martijn Pieters
You are producing a filtered listby using a list comprehension. iis 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.
您正在使用列表理解生成过滤列表。i仍然绑定到该列表的每个元素，并且最后一个元素仍然是'three'，即使它随后从正在生成的列表中过滤掉。
You should not use a list comprehension to pick out one element. Just use a forloop, and breakto end it:
你不应该使用列表推导来挑选一个元素。只需使用一个for循环，并break结束它：
for elem in my_list:
if elem == 'two':
break
If you musthave a one-liner (which would be counter to Python's philosophy, where readability matters), use the next()functionand a generator expression:
如果您必须有一个单行（这与 Python 的哲学背道而驰，可读性很重要），请使用next()函数和生成器表达式：
i = next((elem for elem in my_list if elem == 'two'), None)
which will set ito Noneif there is no such matching element.
如果没有这样的匹配元素，它将设置i为None。
The above is not that useful a filter; your are essentially testing if the value 'two'is in the list. You can use infor that:
上面的过滤器不是那么有用；您本质上'two'是在测试该值是否在列表中。你可以使用in：
elem = 'two' if 'two' in my_list else None
回答by Cong Ma
When you perform
当你执行
>>> [(i) for i in my_list if i=="two"]
iis iterated through the list my_list. As the list comprehension finishes evaluation, iis assigned to the last item in iteration, which is "three".
i遍历列表my_list。当列表理解完成评估时，i被分配给迭代中的最后一项，即"three"。
回答by desiato
In list comprehension the loop variable i becomes global. After the iteration in the for loop it is a reference to the last element in your list.
在列表理解中，循环变量 i 成为全局变量。在 for 循环中迭代之后，它是对列表中最后一个元素的引用。
If you want all matches then assign the list to a variable:
如果您想要所有匹配项，则将列表分配给一个变量：
filtered = [ i for i in my_list if i=='two']
If you want only the first match you could use a function generator
如果你只想要第一场比赛，你可以使用函数生成器
try:
m = next( i for i in my_list if i=='two' )
except StopIteration:
m = None
