列出我从未见过的'i for j in'的迭代

Question

我试图了解以下python代码的作用

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

我从未见过这样的语法，有人可以帮助我吗？

Answer 1

它被称为list comprehension。

使用普通的for循环，其等效代码为：

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

以下是用于展平列表列表的演示：

>>> 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]
>>>