How to modify list entries during for loop?

Since the loop below only modifies elements already seen, it would be considered acceptable:

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

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

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

Which is different from:

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

in that it doesn't require the creation of a temporary list and an assignment of it to replace the original, although it does require more indexing operations.

Caution: Although you can modify entries this way, you can't change the number of items in the list without risking the chance of encountering problems.

Here's an example of what I mean—deleting an entry messes-up the indexing from that point on:

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!

(The result is wrong because it didn't delete all the items it should have.)

Update

Since this is a fairly popular answer, here's how to effectively delete entries "in-place" (even though that's not exactly the question):

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

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

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

See How to remove items from a list while iterating?.

It's considered poor form. Use a list comprehension instead, with slice assignment if you need to retain existing references to the list.

a = [1, 3, 5]
b = a
a[:] = [x + 2 for x in a]
print(b)