What's the difference between the two list comprehension in Python?

Question

I just solved a question in LeetCode, however I found that the two different list comprehensions produce different results. So what’s the difference?

ans = [[0] * (n-2)] * (n-2)

ans = [[0] * (n-2) for _ in range(n-2)]

The question is LeetCode 2373, and the two different results are as follows:
This solution gives the wrong answer because of the list comprehension

class Solution:
    def largestLocal(self, grid: List[List[int]]) -> List[List[int]]:
        n = len(grid)
        ans = [[0] * (n-2)] * (n-2)
        print(ans)
        
        for i in range(n-2):
            for j in range(n-2):
                ans[i][j] = max(grid[x][y] for x in range(i, i+3) for y in range(j, j+3))
        return ans

Asked By: Dog Fish

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Source

Answer 1

[expression] * n evaluates expression once, so you’ll get a list with n occurrences of the same value (and as Iain Shelvington pointed out, this is not a list comprehension). [expression for element in source] reevaluates expression for every element in the source (and this is a list comprehension).

In your situation, this means that the first version will produce a list where each element is the same list of zeroes, so if you set an element in one of the inner lists, you’ll actually set it in all the inner lists.

Answered By: Aasmund Eldhuset

What's the difference between the two list comprehension in Python?

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