How Can I Safely Modify Strings Within a List During Iteration in Python?

Iterating and Modifying Lists Safely

Problem:

Modifying a list during a Python for loop is generally not recommended. However, what if you need to modify the strings within a list? Does replacing these mutable values count as a forbidden modification?

Answer:

As long as you modify elements within the list that have already been iterated over, it is considered acceptable. For example:

a = ['a',' b', 'c ', ' d ']

for i, s in enumerate(a):
    a[i] = s.strip()

print(a) # -> ['a', 'b', 'c', 'd']

This code iterates over the list a and strips whitespace from each string. It does so without creating a new list and assigning it to the original.

However, altering the number of items in the list during iteration is risky. For instance:

b = ['a', ' b', 'c ', ' d ']

for i, s in enumerate(b):
    if s.strip() != b[i]: # leading or trailing whitespace?
        del b[i]

print(b) # -> ['a', 'c '] # WRONG!

This code attempts to delete elements with leading or trailing whitespace from list b. Unfortunately, it fails because deleting elements disrupts the indexing of subsequent iterations.

Caution:

It's important to note that while you can safely modify elements in a list as described, any attempt to increase or decrease the number of elements can lead to erroneous behavior.

In-Place Deletion:

If you want to effectively delete entries during iteration, use list comprehension:

b = ['a',' b', 'c ', ' d ']

b[:] = [entry for entry in b if entry.strip() == entry]

print(b) # -> ['a'] # CORRECT

This code creates a new list containing only the entries with no leading or trailing whitespace, effectively removing the unwanted elements "in-place."

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