Why Does This Python List Loop Produce No Output? A Deep Dive

1. Introduction

In a Python community a user asked about a list‑processing snippet that unexpectedly produced no output. The author shares the original code and explains the issue.

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 code prints nothing because the second if never becomes true during the iteration.

2. Solution Process

A contributor explains that the loop variable i only reaches 6 (seven iterations), so the condition (len(List) - 1) == i is never satisfied.

Annotated version of the first snippet:

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:
        print(List[i] + List[2])

The added prints show that after appending 9 the list length is 8, but i never reaches 7, so the final print never runs.

A variant of the problem iterates directly over the list elements:

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])

Because the list grows during iteration, the second if can become true after the append, leading to different behavior.

3. Summary

The article demonstrates how mutating a list while iterating over it can change loop dynamics and cause unexpected results, such as missing output. By adding detailed comments and intermediate prints, readers can see the list’s length and index values at each step, helping them avoid similar pitfalls in Python programming.