Python Lists: Pitfalls of Item Removal During Iteration

Iterating through a Python list while concurrently removing items can lead to unexpected behavior. A notable example is the following:

letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l']
for i in letters:
    letters.remove(i)

Puzzlingly, the final print of letters reveals that only every other item has been removed.

Reason for the Anomaly

This behavior stems from the way Python handles modifications to iterables during iteration. The documentation explicitly states that modifying a sequence being iterated over is generally unsafe, especially for mutable types like lists.

This practice can lead to undefined behavior and potential changes in future Python builds.

Correct Approach to Remove All Items

To safely remove all items from a list, use any of the following methods:

• del letters[:] to delete all elements and references to the list object.
• letters[:] = [] to assign a new empty list to the existing variable, leaving references to the original object intact.
• letters = [] to create a new empty list and assign it to a new variable.

Handling Conditional Item Removal

For conditional removal of items, create a copy of the list using the [:] slice syntax:

commands = ["ls", "cd", "rm -rf /"]
for cmd in commands[:]:
    if "rm " in cmd:
        commands.remove(cmd)

Alternatively, use the filter function to exclude unwanted items:

commands = [cmd for cmd in commands if not is_malicious(cmd)]