正确使用列表推导-python [英] proper use of list comprehensions - python

问题描述

通常，列表推导用于从现有列表中派生新列表.例如:

Normally, list comprehensions are used to derive a new list from an existing list. Eg:

>>> a = [1, 2, 3, 4, 5]
>>> [i for i in a if i > 2]
[3, 4, 5]

我们应该使用它们执行其他过程吗?例如:

Should we use them to perform other procedures? Eg:

>>> a = [1, 2, 3, 4, 5]
>>> b = []
>>> [b.append(i) for i in a]
[None, None, None, None, None]
>>> print b
[1, 2, 3, 4, 5]

还是应该避免上述情况，而是使用以下内容?

or should I avoid the above and use the following instead?:

for i in a:
    b.append(i)

推荐答案

您确实应该避免使用列表推导(以及字典推导，设置推导和生成器表达式)产生副作用.除了他们会积累虚假列表从而浪费内存这一事实之外，这也令人困惑.我希望列表理解会产生一个(有意义的)值，很多人都会同意.另一方面，循环显然是一系列语句.预计它们将开始产生副作用，并且不会产生结果值-不会感到惊讶.

You should indeed avoid using list comprehensions (along with dictionary comprehensions, set comprehensions and generator expressions) for side effects. Apart from the fact that they'd accumulate a bogus list and thus waste memory, it's also confusing. I expect a list comprehension to generate a (meaningful) value, and many would agree. Loops, on the other hand, are clearly a sequence of statements. They are expected to kick off side effects and generate no result value - no surprise.

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