Python - list[list] * int operation

 

>>> m, n = 3, 4
>>> grid = [[float('inf')] * n] * m
>>> grid
[[inf, inf, inf, inf], [inf, inf, inf, inf], [inf, inf, inf, inf]]
>>> for j in range(n):
...     grid[0][j] = 1
...     
>>> grid
[[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]
>>> grid = [[float('inf')] * n for _ in range(m)]
>>> grid
[[inf, inf, inf, inf], [inf, inf, inf, inf], [inf, inf, inf, inf]]
>>> for j in range(n):
...     grid[0][j] = 1
...     
>>> grid
[[1, 1, 1, 1], [inf, inf, inf, inf], [inf, inf, inf, inf]]