Create a list of lists of lists in a single line

Question

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?

I'mahdi · Accepted Answer · 2022-12-23T13:37:07.873

9

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)

edited Dec 23 '22 at 13:37

answered Dec 23 '22 at 13:10

I'mahdi

Why NumPy instead of simply `res == vals`? – Kelly Bundy Dec 23 '22 at 13:38
@KellyBundy, I want to show that both of them same shape. Yes we can use `res == vals` – I'mahdi Dec 23 '22 at 13:42

score 4 · Answer 2 · answered Dec 23 '22 at 13:10

4

Using list comprehension:

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

answered Dec 23 '22 at 13:10

dm2

score 1 · Answer 3 · answered Dec 23 '22 at 13:37

1

numpy way:

import numpy as np

vals = np.zeros((25,25,0)).tolist()

answered Dec 23 '22 at 13:37

jvx8ss

score 0 · Answer 4 · answered Dec 23 '22 at 13:34

0

You can also use this approach based on map:

vals = list(map(lambda _: list(map(lambda __: [], range(25))), range(25)))

answered Dec 23 '22 at 13:34

Melchia

score -1 · Answer 5 · answered Dec 23 '22 at 13:23

-1

Just a fun hack for lazy people:

vals = eval(repr([[[]] * 25] * 25))

answered Dec 23 '22 at 13:23

Kelly Bundy

5 Answers5