Why Iterating and Removing from a List in Python Can Cause Unexpected Results

Removing Items from a List During Iteration - Why the Typical Idiom Fails

In programming, it's common practice to iterate over a list and remove items as needed. However, when it comes to Python, there's a caveat to this idiom that can cause unexpected results.

Consider the following Python code:

<code class="python">letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l']
for i in letters:
    letters.remove(i)
print(letters)</code>

This code attempts to remove all items from a list. Surprisingly, the output of the code is not an empty list, but instead contains every other item from the original list:

['b', 'd', 'f', 'h', 'j', 'l']

Explanation

The reason for this behavior lies in Python's handling of list modification during iteration. The documentation clearly states:

"It is not safe to modify the sequence being iterated over in the loop [...] If you need to modify the list you are iterating over [...] you must iterate over a copy."

In the provided code, we are attempting to modify the list letters while iterating over it. Python's design for handling this situation is that the iteration proceeds with skipping every other item after each removal.

Rewriting the Code

To avoid this issue, the code must be rewritten to create a copy of the list before iterating over it. There are several methods to accomplish this:

• del letters[:]: Removes all items from the list, effectively replacing it with an empty list.
• letters[:] = []: Similar to del letters[:], but generates a new empty list instead of modifying the original.
• letters = []: Assigns a completely new empty list to the variable letters.

Alternatively, if the goal is to remove specific items based on a condition, it's more efficient to use the filter() function to create a new list containing only the desired items. This approach avoids the need for iteration over a copy of the original list.

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