Nested list multiplication trap

Question

Accepted Answer

The * 2 operator does not create two independent inner lists - it creates two references to the same inner list. So grid[0] and grid[1] point at the same object, and mutating one mutates both. To build a genuine 2D grid, use a list comprehension: [[0] * 3 for _ in range(2)]. This is one of the most common Python gotchas in coding interviews and data pipeline code alike.