Python列表+列表vs. list.append() [英] Python list + list vs. list.append()

问题描述

今天我花了大约20分钟试图弄清楚为什么 这按预期工作:

Today I spent about 20 minutes trying to figure out why this worked as expected:

users_stories_dict[a] = s + [b] 

，但这将具有None值:

users_stories_dict[a] = s.append(b)

任何人都知道为什么append函数不返回新列表吗?我正在寻找做出此决定的明智原因；现在看来，这对我来说就像是Python新手.

Anyone know why the append function does not return the new list? I'm looking for some sort of sensible reason this decision was made; it looks like a Python novice gotcha to me right now.

推荐答案

append实际是通过修改列表来完成的，所以所有的魔术都是副作用.因此，append返回的结果为None.换句话说，一个人想要的是:

append works by actually modifying a list, and so all the magic is in side-effects. Accordingly, the result returned by append is None. In other words, what one wants is:

s.append(b)

然后:

users_stories_dict[a] = s

但是，您已经知道了很多.至于为什么这样做，虽然我真的不知道，但我的猜测是它可能与0(或false)退出值有关，该退出值指示操作正常进行，并通过返回None对于作用是就地修改其参数的函数，您报告修改成功.

But, you've already figured that much out. As to why it was done this way, while I don't really know, my guess is that it might have something to do with a 0 (or false) exit value indicating that an operation proceeded normally, and by returning None for functions whose role is to modify their arguments in-place you report that the modification succeeded.

但是我同意，如果它返回修改后的列表会很好.至少，Python的行为在所有此类函数中都是一致的.

But I agree that it would be nice if it returned the modified list back. At least, Python's behavior is consistent across all such functions.

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