列出具有多个条件的理解(python) [英] list comprehension with multiple conditions (python)

问题描述

以下代码在Python中有效

The following code works in Python

var=range(20)
var_even = [0 if x%2==0 else x for x in var]
print var,var_even

但是，我认为条件必须放在列表的末尾.如果我编写代码

However, I thought that the conditions need to be put in the end of a list. If I make the code

var_even = [0 if x%2==0 for x in var]

然后它将不起作用.有这个原因吗?

Then it won't work. Is there a reason for this?

推荐答案

这里涉及两种截然不同但外观相似的语法，条件表达式和列表理解过滤器子句

There are two distinct but similar-looking syntaxes involved here, conditional expressions and list comprehension filter clauses.

条件表达式的格式为x if y else z.此语法与列表推导无关.如果您想在列表理解中有条件地包含一件事或另一件事，那么您将使用以下方法:

A conditional expression is of the form x if y else z. This syntax isn't related to list comprehensions. If you want to conditionally include one thing or a different thing in a list comprehension, this is what you would use:

var_even = [x if x%2==0 else 'odd' for x in var]
#             ^ "if" over here for "this or that"

列表理解过滤子句是elem for x in y if thing中的if thing.这是列表理解语法的一部分，并且在for子句之后.如果要在列表理解中有条件地包括或不包括元素，则可以使用以下方法:

A list comprehension filter clause is the if thing in elem for x in y if thing. This is part of the list comprehension syntax, and it goes after the for clause. If you want to conditionally include or not include an element in a list comprehension, this is what you would use:

var_even = [x for x in var if x%2==0]
#                          ^ "if" over here for "this or nothing"

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