Why Removing Items from a Python List Can Yield Unexpected Results—and How to Avoid Them

Introduction

Hello, I’m a Python enthusiast. Recently I shared a question about deleting items from a Python list in a community chat, and I’m posting the discussion here for everyone to learn from.

lst = ['鲁班', '鲁大师', '鲁班大师', '鲁智深']
for i in lst:
    if i.startswith('鲁'):
        lst.remove(i)
print(lst)

What does this code output?

What Actually Happens

At first glance you might expect an empty list, but the result is ['鲁大师', '鲁智深']. The reason is that removing items while iterating mutates the list and shifts the internal pointer, causing some elements to be skipped.

Step‑by‑step:

First iteration: i = '鲁班' is removed, list becomes ['鲁大师', '鲁班大师', '鲁智深'].

The pointer moves to the next index, which now points to '鲁班大师' (originally the third element). It is removed, leaving ['鲁大师', '鲁智深'].

The pointer moves again, but the next position is beyond the current list length, so the loop ends.

How to Avoid This Pitfall

Directly deleting items from a list during iteration is not recommended. Here are several safe alternatives:

Create a copy of the list and build a new list with the desired elements.

Use a shallow copy: for i in lst[:].

Apply filter with a lambda function, e.g., list(filter(lambda x: not x.startswith('鲁'), lst)).

Use list comprehensions to construct a filtered list.

Conclusion

This article highlighted a subtle bug when removing items from a Python list during iteration and provided multiple robust methods to perform deletions safely. Understanding this behavior helps avoid unexpected results in future Python code.