Simple python code about double loop

Question

I tested the following python code on Spyder IDE. Thinking it would output 2d array q as increasing number as 0..31 from q[0][0] to q[3][7]

Answer 1

q=[somelist]*4 

creates a list with four identical items, the list somelist. So, for example, q[0] and q[1] reference the same object.

Thus, in the nested for loop q[i] is referencing the same list regardless of the value of i.

To fix:

q = [[0]*8 for _ in range(4)]

The list comprehension evaluates [0]*8 4 distinct times, resulting in 4 distinct lists.

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Here is a quick demonstration of this pitfall:

In [14]: q=[[0]*8]*4

You might think you are updating only the first element in the second row:

In [15]: q[1][0] = 100

But you really end up altering the first element in every row:

In [16]: q
Out[16]: 
[[100, 0, 0, 0, 0, 0, 0, 0],
 [100, 0, 0, 0, 0, 0, 0, 0],
 [100, 0, 0, 0, 0, 0, 0, 0],
 [100, 0, 0, 0, 0, 0, 0, 0]]