Create a list of lists of lists in a single line

This code creates a list of 25 lists of 25 lists:

vals = []
for i in range(25):
    vals.append([])
    for j in range(25):
        vals[i].append([])

How could I translate this code to a single line instead of using 5 lines in Python?

>Solution :

You can use list_comprehension.

res = [[[] for _ in range(25)] for _ in range(25)]

To check that result is the same, we can use numpy.ndarray.shape.

>>> import numpy as np
>>> np.asarray(vals).shape
(25, 25, 0)

>>> np.asarray(res).shape
(25, 25, 0)