Why This Python List Loop Prints Nothing – Uncovering a Common Pitfall

1. Introduction

A fan named Chloe asked a question about a Python list loop in a community chat, and the author shares the problem and solution for everyone to learn.

List = [1, 2, 4, 5, 6, 7, 8]
for i in range(0, len(List)):
    if List[i] % 5 == 0:
        List.append(9)
    if (len(List) - 1) == i:
        print(List[i] + List[2])

The original code produces no output because the second if condition never becomes true during the loop.

2. Solution Process

Another contributor, 月神, explained that the for i loop iterates only up to index 6 (seven iterations). The following annotated version clarifies the execution flow:

List = [1, 2, 4, 5, 6, 7, 8]
for i in range(0, len(List)):
    if List[i] % 5 == 0:  # when List[i] == 5 (i = 3)
        List.append(9)
        print(f"Current length is {len(List)}")  # length becomes 8
        print(List)
    print(f"Current List[i] is: {List[i]}")
    print(f"Current i is: {i}")
    print(f"Current len is: {len(List)}")
    if (len(List) - 1) == i:  # never true after appending 9
        print(List[i] + List[2])

A second variant of the problem was also shared:

List = [1, 2, 4, 5, 6, 7, 8]
for i in List:
    if i % 5 == 0:
        List.append(9)
    if (List[len(List) - 1]) == i:
        print(i + List[2])

In this version, the loop iterates over the list elements directly. After appending 9, the list grows, and the second if may become true, leading to different behavior.

3. Summary

The article demonstrates how modifying a list while iterating over it can change loop boundaries and cause unexpected results. By adding detailed comments and printing intermediate states, the author clarifies why the original code prints nothing and how the variant behaves.