Making 2D arrays: Don't use [[0] * arr_len] * arr_len

Learned that when using python

[[0] * width] * height

Is wrong and

[[0 for _ in range(width)] for __ in range(height)]

Is correct to make 2d arrays

Code testing making 2D lists

As you can see when the first_arr[1][2] is set to '1' all the other rows are affected too.

I think this is because when

first_arr = [[0] * width] * height

is inputed, first a row with 'width' number of '0' is made. Then the same list is made as an instance to the array.

 

While searching online I learned that 

first_arr = [[0] * width for _ in range(height)]

Is the fastest way to make a 2D array.