Is Modifying List Entries During Iteration Safe, and How Can I Do It Correctly?

Modifying List Entries During Loop Iteration

It is generally not advisable to modify a list during iterative looping. However, when working with lists of strings that require string manipulation, one may wonder if replacing the strings within the list constitutes a modification.

Is In-Place Value Mutation Modification?

The following approach is deemed acceptable because it modifies elements after they have been processed:

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

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

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

Contrast this with the following approach:

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

The latter requires creating a temporary list and assigning it, which can be computationally expensive compared to the in-place modification.

Caution: Updating Length

While in-place value mutation is permitted, changing the length of the list can lead to problems. Deleting elements during iteration, for example, can result in incorrect indexing:

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

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

print(b) # ['a', 'c '] # Incorrect result

Effective Deletion During Loop

To effectively delete entries in-place during iteration, consider the following approach:

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

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

print(b) # ['a'] # Correct result

In conclusion, while it is possible to modify list entries during a for loop through in-place value mutation, it is important to exercise caution when modifying the list's length to avoid unexpected consequences. Deleting entries, in particular, requires special handling to maintain the integrity of the iteration.

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