Python 将值附加到空列表的最佳实践
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Best practice to append value to an empty list
提问by ferdy
I'm just curious about the following code fragment not working, which should initialize an empty list and on-the-fly append an initial value. The second fragment is working. What's wrong with it? Is there a best practice for such initializing?
我只是对以下代码片段不工作感到好奇，它应该初始化一个空列表并即时附加一个初始值。第二个片段正在工作。它出什么问题了？这种初始化有最佳实践吗？
>>> foo = [].append(2)
>>> print foo
None
>>> foo = []
>>> foo.append(2)
>>> print foo
[2]
EDIT: Seems I had a misunderstanding in the syntax. As already pointed out below, append always returns None as a result value. What I first thought was that [] would return an empty list object where the append should put a first element in it and in turn assigns the list to foo. That was wrong.
编辑：似乎我对语法有误解。正如下面已经指出的， append 总是返回 None 作为结果值。我首先想到的是 [] 将返回一个空列表对象，其中 append 应该在其中放置第一个元素，然后将列表分配给 foo。那是错误的。
采纳答案by Alexander McFarlane
The actual reason why you can't do either of the following,
您不能执行以下任一操作的实际原因，
l = [].append(2)
l = [2,3,4].append(1)
is because .append()always returns Noneas a function return value. .append()is meant to be done in place.
是因为.append()总是None作为函数返回值返回。.append()旨在就地完成。
See herefor docs on data structures. As a summary, if you want to initialise a value in a list do:
有关数据结构的文档，请参见此处。总而言之，如果要初始化列表中的值，请执行以下操作：
l = [2]
If you want to initialise an empty list to use within a function / operation do something like below:
如果要初始化一个空列表以在函数/操作中使用，请执行以下操作：
l = []
for x in range(10):
value = a_function_or_operation()
l.append(value)
Finally, if you really want to do an evaluation like l = [2,3,4].append(), use the +operator like:
最后，如果你真的想做一个像 那样的评估l = [2,3,4].append()，使用+像这样的运算符：
l1 = []+[2]
l2 = [2,3,4] + [2]
print l1, l2
>>> [2] [2, 3, 4, 2]
This is generally how you initialise lists.
这通常是您初始化列表的方式。
