嵌套列表理解

与嵌套循环的列表推导不同，嵌套列表推导是列表推导中的列表推导。初始表达式可以是任意表达式，包括另一个列表理解。

#List Comprehension with nested loop
[x + y for x in [1, 2, 3] for y in [3, 4, 5]]
#Out: [4, 5, 6, 5, 6, 7, 6, 7, 8]

#Nested List Comprehension
[[x + y for x in [1, 2, 3]] for y in [3, 4, 5]]
#Out: [[4, 5, 6], [5, 6, 7], [6, 7, 8]]

嵌套示例等同于

l = []
for y in [3, 4, 5]:
    temp = []
    for x in [1, 2, 3]:
        temp.append(x + y)
    l.append(temp)

可以使用嵌套理解来转置矩阵的一个示例。

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

[[row[i] for row in matrix] for i in range(len(matrix))]
# [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

就像嵌套的 for 循环一样，嵌套的深度理解也没有限制。

[[[i + j + k for k in 'cd'] for j in 'ab'] for i in '12']
# Out: [[['1ac', '1ad'], ['1bc', '1bd']], [['2ac', '2ad'], ['2bc', '2bd']]]