Create an empty list of lists in python

Question

I would like to create in python a list that inside has 3 lists, each of them have 2 vectors of 4 elements, something like that:

[[[0,0,0,0],[0,0,0,0]], 
 [[0,0,0,0],[0,0,0,0]], 
 [[0,0,0,0],[0,0,0,0]]]

This is just a simple example but I would like to automate it in order to be able to create an object like that with a much higher number of elements.

I tried to do it the following way:

grid=[np.zeros((2,4)) for x in range(3)]
grid

However when I print it the result is something like this instead

[array([[0., 0., 0., 0.],
        [0., 0., 0., 0.]]), array([[0., 0., 0., 0.],
        [0., 0., 0., 0.]]), array([[0., 0., 0., 0.],
        [0., 0., 0., 0.]])]

The structure should be fine, but I don´t know if the fact that it says array is normal or I did something wrong.

Thanks in advance for your help

Answer 1

You have here a list of 2D np.array , if you want a list of list of lists :

grid = [[[0 for _ in range(4)] for _ in range(2)] for _ in range(3)]

The following will build the same structure, but each sublist will be copy, so unusable (see List of lists changes reflected )

grid = [[[0] * 4] * 2] * 3