理解列表理解 [英] Comprehending List Comprehensions

问题描述

最近我被介绍列出理解，但是，我不确定我是否完全理解它们.

I have been recently introduced to list comprehensions, however, I am not certain I understand them completely.

因此，我通过文档和Google进行了快速搜索. 我在数据结构上找到了docs部分.

So, I did a quick search through the docs and google. I found the docs section over data structures.

https://docs.python.org/2/tutorial/datastructures.html

虽然让我对它们的工作方式有所了解，但让我感到陌生，但我想对它们的运行方式有更成熟的了解.

While that gives me an idea with how they work, call me strange, I want to have a more mature understanding of how they operate.

这是文档中的一个示例:

Here is an example from the docs:

>>>[(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]

如果我错了，请纠正我，但这是我了解列表评估方式的方式.

Correct me if I am wrong, but this is how I understand how the list is evaluating.

• 列表理解似乎是从左到右评估的.
• 这种理解引入了X和Y.
• 然后通过列表定义X和Y，并根据列表中的位置以及if条件进行评估.
• 由于将X分配为第一个变量，就像嵌套的for循环的外部一样，因此将其首先调用并针对Y的列表进行迭代.
• 如果条件满足，则理解仅会产生与X遍历Y的列表不相似的对.

如果我的理解不正确，请纠正我. 我欢迎更复杂的理解实例.

Please correct me if my understanding is incorrect. I welcome more complex examples for comprehensions.

推荐答案

嵌套列表推导没有任何魔术.我认为您还没有看到它们的简单性！

There's nothing magic about nested list comprehensions. I think you're simply not seeing their simplicity yet!

此:

#      X           A                   B               C
a = [(x, y)   for x in [1,2,3]   for y in [3,1,4]   if x != y]

仅仅是以下方面的语法糖:

is simply syntactic sugar for:

__tmp = []
for x in [1,2,3]:                   # A
    for y in [3,1,4]:               # B
        if x != y:                  # C
            __tmp.append((x, y))    # X

a = __tmp

因此，您将for ... in ...零件重新组装成嵌套的for循环.然后，您将if放入相应的for循环中；最后，使用第一部分(在本例中为(x, y))，然后嵌套在嵌套的for结构的最深部分.

So you take the for ... in ... parts and re-assemble them as nested for loops. You then take the ifs and nest it in the corresponding for loop; finally, you take the first part ((x, y) in this case) and you nest at the deepest part of the nested for structure.

当if出现在列表理解中的多个阶段"时，这也可以正常工作，例如:

This works out fine also when ifs appear at multiple "stages" in a list comprehension, for example:

#      X         A                 B               C               D
a = [(x, y)   for x in [1,2,3]   if x >= 2   for y in [3,1,4]   if x != y]

将(反义词)翻译为:

__tmp = []
for x in [1,2,3]:                     # A
    if x >= 2:                        # B
        for y in [3,1,4]:             # C
            if x != y:                # D
                __tmp.append((x, y))  # X

a = __tmp

再说一遍……这与散布在各个部分之间的冒号的理解完全相同，而内部部分则位于前面. A B C D ...部分的顺序始终相同，并且从未更改.只是X部分从后向前移动.

So again... it's just exactly the same as the comprehension with colons interspersed between the parts, and the inner part brought right in the front. The order of the A B C D ... part is always the same and never changed; it's only the X part that's moved from the back to the front.

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