That One Python List Bug That Wasted My Time 😅

Source: Dev.to

The Issue

I recently ran into a weird Python bug while solving a problem that needed a 2‑D list.
The code looked correct, but the output was completely wrong. The culprit was this line:

index = [[0, 0]] * n

If you’ve written Python for a while, you might expect this to create n rows, each with [0, 0], like:

[
  [0, 0],
  [0, 0],
  [0, 0]
]

Why * Doesn’t Work Here

Python doesn’t create n separate inner lists. It creates one list [0, 0] and references it n times. All rows point to the same object.

Proof

index = [[0, 0]] * 3
index[0][0] = 7
print(index)

Output

[[7, 0], [7, 0], [7, 0]]

Changing one value changes every row because the * operator performs a shallow copy, effectively doing:

index = [same_list, same_list, same_list]

The Correct Way

Use a list comprehension to create a new inner list for each row:

index = [[0, 0] for _ in range(n)]

Quick Check

index = [[0, 0] for _ in range(3)]
index[0][0] = 7
print(index)

Output

[[7, 0], [0, 0], [0, 0]]

Now only the intended row is modified.

When * Is Actually Fine

Using * is perfectly acceptable with immutable values:

arr = [0] * 5   # perfectly fine

The problem only arises when you use it with nested mutable objects like lists.