Now I know that it is not safe to modify the list during an iterative looping. However, suppose I have a list of strings, and I want to strip the strings themselves. Does replacement of mutable values count as modification?
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)
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.)
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