List items are not changed in for loop

Question

I was shocked when the following code did not do what I expected it to do:

lines_list = ['this is line 1\n', 'this is line 2\n', 'this is line 3\n']
for line in lines_list:
    line = line.strip()

In Perl or in Java, my usage works well. Python is the one with the unique behavior.

In Java this will work:

String[] lines = {"this is line 1\n", "this is line 2\n", "this is line 3\n"};
for (String line : lines) {
    line = line.trim();
}

In Perl this will work:

my @lines = ("this is line 1\n", "this is line 2\n", "this is line 3\n");
foreach my $line (@lines) {
    $line =~ s/\s+$//;
    $line =~ s/^\s+//;
}

I of course expected each item in the list to become "stripped", i.e. in this case, to be free of the trailing '\n' char, but it did not...

print lines_list

The output:

['this is line 1\n', 'this is line 2\n', 'this is line 3\n']

Is there an elegant way to change the list items during the for loop? I don't want to duplicate the list...

five years later, but the java example doesn't modify the original `lines` either, for the same reason the python example doesn't https://repl.it/@RyanHaining1/UntruePowderblueDifference — Ryan Haining, Jun 19 '19 at 18:46
Also five years later, it really look strange to me. Since String is immutable, I'd not expect `line = line.trim()` to affect the reference that's in the array — SomethingSomething, Jun 20 '19 at 10:26
it's the same reason because you're changing `line` to reference a new string. Even with a mutable type, `=` won't change the object on the left side, `[]=` is different. — Ryan Haining, Jun 20 '19 at 18:56
Does this answer your question? [Can't modify list elements in a loop](https://stackoverflow.com/questions/19290762/cant-modify-list-elements-in-a-loop) — mkrieger1, Jan 23 '22 at 19:48

Ryan Haining · Accepted Answer · 2023-08-01T20:39:21.490

5

Your initial claim about python's uniqueness is incorrect. If you run the Java code then print the contents of lines you will see that the array contains the original unstripped strings, for the same reason it does it python.

You can go through by index and modify in place that way

for i, _ in enumerate(lines_list):
    lines_list[i] = lines_list[i].strip()

though I think many would prefer the simplicity of duplicating the list if the list isn't so big that it causes an issue

lines_list = [line.strip() for line in lines_list]

The issue is using the = operator reassigns to the variable line, it doesn't do anything to affect the contents of the original string. New python programmers are equally as often surprised when:

for i in range(10):
    print(i)
    i += 1

prints the numbers 0,1,2,3,4,5,6,7,8,9. This occurs because the for loop reassigns i at the beginning of each iteration to the next element in the range. It's not exactly your problem, but it's similar.

Since you are reading lines out of a file, and stripping them afterwards, really what you should do is

with open(file_name) as f:
    lines_list = [line.strip() for line in f]

which reads and strips one line at a time, rather than reading everything first, then stripping the lines afterwards

edited Aug 01 '23 at 20:39

answered May 11 '14 at 20:28

Ryan Haining

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I agree, but I think it's less elegant. Also, I guess (but have no proof) that accessing the list items using indexes instead of an iterator is more expensive in performances – SomethingSomething May 11 '14 at 20:30
The new solution you added is much more beautiful in my opinion. Thanks! – SomethingSomething May 11 '14 at 20:31
@user3322273 curious, are you getting these lines by reading in a file into a list? – Ryan Haining May 11 '14 at 20:32
Ryan's answer is cool. Generally (depending on memory restrictions you might have) it is not good practice to change the collection you are iterating over as you iterate (in fact some languages, like C#, forbid it and will through a compile-time error) as it introduces (sometimes) avoidable complications (each iteration is working with a modified collection). – orcaman May 11 '14 at 20:34
Yes, but I didn't want people go pay attention to file reading, so I showed here an hard-coded list. But it's the same – SomethingSomething May 11 '14 at 20:34
I accepted the answer. You can take a look on the edit I made in the question, I think it could interest you – SomethingSomething May 12 '14 at 06:48

score 3 · Answer 2 · answered May 11 '14 at 20:33

You did forget to assigned new value (stripped string) to something.

Another way, how to assign it to a list in shorter way is:

>>> lst = ['this is line 1\n', 'this is line 2\n', 'this is line 3\n']
>>> import string
>>> lst = map(string.strip, lst)
>>> lst
['this is line 1', 'this is line 2', 'this is line 3]

score 1 · Answer 3 · answered May 11 '14 at 20:31

In your example, line is simply a variable. It is not connected to the list, where it might come from. Think of the problems, that would occur, if every variable modifies its origin!

In python, lists normally are not modified but a new one is created. Therefore there is something called list comprehension:

lines_list = [line.strip() for line in lines_list]

List items are not changed in for loop

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