Python: How do I use tuples in lists?

Question

I have this code that doesn't work the way I expect it to. I'm trying to make a grid. It should output the tuple from the list, but it's stuck on the first number.

Is there a problem with my logic?

row = []
grid = []

for y in range(0,3):
    row.append((0,0))

for x in range(0,3):
    grid.append(row)


for y in range(0, 3):
    for x in range (0, 3):
        print (x,y)
        grid[x][y] = (x,y)
        print (grid[x][y])

print ("\nTesting:")
for y in range(0, 3):
    for x in range (0, 3):
        print (grid[x][y])

What it outputs is this:

0 0
(0, 0)
1 0
(1, 0)
2 0
(2, 0)
0 1
(0, 1)
1 1
(1, 1)
2 1
(2, 1)
0 2
(0, 2)
1 2
(1, 2)
2 2
(2, 2)

Testing:
(2, 0)
(2, 0)
(2, 0)
(2, 1)
(2, 1)
(2, 1)
(2, 2)
(2, 2)
(2, 2)

Ah, the old mutable variable gotcha. Let me find the dupe link. — SuperStormer, Aug 29 '21 at 01:01
Related, but not quite a dupe: https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly https://stackoverflow.com/questions/2612802/list-changes-unexpectedly-after-assignment-why-is-this-and-how-can-i-prevent-it — SuperStormer, Aug 29 '21 at 01:05

score 3 · Answer 1 · answered Aug 29 '21 at 01:04

The problem is this:

for x in range(0,3):
    grid.append(row)

Keep in mind that row is the name that refers to a particular list. By executing this line of code multiple times, you're adding the same list to grid multiple times. In other words, grid[0], grid[1], and grid[2] are all pointing to the same piece of memory.

As such, when you modify the list that all three are pointing at, then all three will display the changes.

To avoid this you need to make a copy of the list when inserting it, so you're not inserting the same object multiple times (just copies of the same object). The easiest way to do this is to make a blank slice:

for x in range(0, 3):
    grid.append(row[:])

Ahh! I knew there was some trick! Thank you! – Arnaldo Montoya Aug 29 '21 at 01:07 — Arnaldo Montoya, Aug 29 '21 at 01:07

U13-Forward · Answer 2 · 2021-08-29T01:22:30.313

0

It's because tuples are hashable, so you would have to copy those objects:

for x in range(0, 3):
    grid.append(row.copy())

edited Aug 29 '21 at 01:22

answered Aug 29 '21 at 01:09

U13-Forward

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Tuples are not mutable. `Row` is a list, and lists are mutable. – Acccumulation Aug 29 '21 at 01:21
@Acccumulation Sorry I meant hashable, and also immutable, it was my typo – U13-Forward Aug 29 '21 at 01:22

Python: How do I use tuples in lists?

2 Answers2