CODEWITHSUNDEEP
pythonlisttuples
Arnaldo Montoya
Arnaldo Montoya

Reputation: 29

Python: How do I use tuples in lists?

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)

Upvotes: 1

Views: 52

Answers (2)

U13-Forward
U13-Forward

Reputation: 71570

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

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

Upvotes: 0

Green Cloak Guy
Green Cloak Guy

Reputation: 24691

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[:])

Upvotes: 3

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