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]. But it actually returns q as:

[[24, 25, 26, 27, 28, 29, 30, 31], [24, 25, 26, 27, 28, 29, 30, 31], [24, 25, 26, 27, 28, 29, 30, 31], [24, 25, 26, 27, 28, 29, 30, 31]]. 

The code:

q=[[0]*8]*4 
for i in range(4): 
    for j in range(8): 
        q[i][j] = 8*i+j 
print q

Any idea of what's happening here? I debugged step by step. It shows the updates of every row will sync with all other rows, quite different from my experience of other programing languages.

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.

---

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]]

Answer 2

As explained the problem is caused due to * operation on lists, which create more references to the same object. What you should do is to use append:

q=[]
for i in range(4): 
    q.append([])
    for j in range(8): 
        q[i].append(8*i+j)
print q 

[[0, 1, 2, 3, 4, 5, 6, 7], [8, 9, 10, 11, 12, 13, 14, 15], [16, 17, 18, 19, 20, 21, 22, 23], [24, 25, 26, 27, 28, 29, 30, 31]]