Asked by Zoë Achterberg · 725 views · 2 replies
Making a grid for a tic-tac-toe assignment:
grid = [[0] * 3] * 3
grid[0][0] = "X"
print(grid)
# [['X', 0, 0], ['X', 0, 0], ['X', 0, 0]]I only set one cell. Why did it end up in every row?
Because you do not have three rows. You have one row, listed three times.
[0] * 3 builds one list. Then [that_list] * 3 makes an outer list holding three references to the same inner list. Change it through any of them and every "row" shows the change, because they are all the same object. You can check: grid[0] is grid[1] prints True.
Build a new inner list for each row:
grid = [[0] * 3 for _ in range(3)]The comprehension runs [0] * 3 again on every pass, so each row is its own list. The inner * 3 is fine, by the way: integers are immutable, so sharing them never causes this.
The is check is what made it click. Fixed, and my win detection suddenly works too, which in hindsight was the same bug.