合并列表中的元素:好像python以两种不同的方式对待同一个项目，我不知道为什么 [英] Combining elements in list: seems like python treats the same item in two different ways and I don't know why

问题描述

我正在通过CodeAcademy进行工作，并且我有一个未解决的问题.任务是获取列表列表，并为其所有元素制作一个列表.下面的代码是我的工作的答案.但是我不明白的是为什么项目"被视为该代码列表中的元素，而(请参见下面的注释)...

I'm working my way through CodeAcademy and I have a question that's going unanswered there. The assignment is to take a list of lists and make a single list of all its elements. The code immediately below is my answer that worked. But what I don't understand is why "item" is treated as elements in a list for that code whereas (see comment continued below)...

m = [1, 2, 3]
n = [4, 5, 6]
o = [7, 8, 9]

def join_lists(*args):
    new_list = []
    for item in args:        
        new_list += item
    return new_list


print join_lists(m, n, o)

...下面的代码中的项目"被视为整个列表，而不是列表中的元素.下面的代码给出了输出:

...the "item" in the code below is treated as the whole list instead of elements in a list. The code below gives the ouput:

 [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

我还尝试使用: new_list.append(item [0:] [0:])，认为它会遍历索引和子索引，但结果相同.我只是不明白这是怎么解释的.

I also tried to use: new_list.append(item[0:][0:]) thinking it would iterate through the index and sub-index but it gave the same result. I just don't understand how this is being interpreted.

m = [1, 2, 3]
n = [4, 5, 6]
o = [7, 8, 9]


def join_lists(*args):
    new_list = []
    for item in args:        
        new_list.append(item)
    return new_list


print join_lists(m, n, o)

此外，我知道我可以在上面的代码中添加另一个for循环，并且我知道为什么可以这样做，但是我仍然不明白为什么Python会以不同的方式解释它们.

Also, I know I could add another for-loop to the code above, and I get why that works, but I still don't understand with one line of difference why Python interprets these differently.

推荐答案

列表中的+=就地添加运算符与调用

The += in-place add operator on a list does the same thing as calling list.extend() on new_list. .extend() takes an iterable and adds each and every element to the list.

list.append()将单个项添加到列表中.

>>> lst = []
>>> lst.extend([1, 2, 3])
>>> lst
[1, 2, 3]
>>> lst.append([1, 2, 3])
>>> lst
[1, 2, 3, [1, 2, 3]]

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