Changing one row of a 2D list changes them all

[[0] * cols] * rows repeats a reference to one inner list. Every row is the same object, so one assignment shows up everywhere.

PythonMutation and copying bugs

What it means

Multiplying a list copies references, not values. For numbers that is harmless, because numbers cannot be changed in place, but for an inner list it means grid[0] and grid[1] are one list seen through two names.

Common causes

1. Building a grid with *

The inner list is created once and shared.

Breaks

grid = [[0] * 3] * 3
grid[0][0] = 1   # every row changes

Works

grid = [[0] * 3 for _ in range(3)]

2. Repeating a dict or list default

Same trap with any mutable value.

Breaks

rows = [{}] * 5

Works

rows = [{} for _ in range(5)]

3. Copying a grid with a shallow copy

copy() and [:] copy the outer list only.

Breaks

backup = grid[:]

Works

import copy
backup = copy.deepcopy(grid)

How to find it in your own code

Build each inner list separately with a comprehension. When a change appears in several places at once, print id() of the parts: matching ids mean shared objects.

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Other common errors

list.sort() returns None TypeError: 'str' object does not support item assignment RuntimeError: dictionary changed size during iteration Mutable default argument keeps old values Array(n).fill([]) shares one array IndexError: list index out of range IndexError: string index out of range AttributeError: 'NoneType' object has no attribute