Why does assigning to one element of a list result in two mutations?

Asked Nov 21 '22 at 23:59

Active Nov 22 '22 at 00:05

Viewed 19 times

This code block is giving me unexpected results, and I am not sure why:

grid = [[0] * cols] * rows
grid[0][1] = 1
print(grid) # [[0, 1], [0, 1]]

I expected the grid to be [[0, 1], [0, 0]], not [[0, 1], [0, 1]]. What causes grid[1][1] to be mutated as well?

Thank you.

edited Nov 22 '22 at 00:05

asked Nov 21 '22 at 23:59

Andre

1

There is a *single mutation* here. You have *one* list in `grid` repeated `row` number of times. Every row **is the exact same list object** – juanpa.arrivillaga Nov 22 '22 at 00:05
`grid = [[0] * cols] * rows` This creates many references to _the same inner list object_. – John Gordon Nov 22 '22 at 00:05
The answer, by the way, is to do `[[0] * cols for _ in rows]`. – Tim Roberts Nov 22 '22 at 00:11

0 Answers0