我从未见过的“i for j in”的列表迭代

Question

我正在尝试理解下面这段Python代码的作用

plain_list = [ j for i in arguments for j in i ]

我从来没见过这样的语法，有人能帮我解释一下吗？

Answer 1

这叫做列表推导式。

如果用普通的for循环来写，代码会是这样的：

plain_list = []               # Make a list plain_list
for i in arguments:           # For each i in arguments (which is an iterable)
    for j in i:               # For each j in i (which is also an iterable)
        plain_list.append(j)  # Add j to the end of plain_list

下面是一个示例，展示了如何用它来把一个列表中的列表变成一个平坦的列表：

>>> arguments = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>>
>>> plain_list = [ j for i in arguments for j in i ]
>>> plain_list
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>
>>> plain_list = []
>>> for i in arguments:
...     for j in i:
...         plain_list.append(j)
...
>>> plain_list
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>