Python: strange issue where conditionally removing elements from a list

Question

(I see that this question is a duplicate. Sorry everybody, I wasn't sure of the wording so I couldn't find the other question)

I define the following list of strings:

x=["foo_a","foo_a_b","foo"]

then I do the following:

for i in x:
    if (("a" not in i) or ("b" in i)):
        x.remove(i)

and I get

In [92]: x
Out[92]: ['foo_a', 'foo']

I would expect the element "foo" to be removed too, because it doesn't contain "a" and so it doesn't verify the first condition.

What am I missing?

You should not modify a list while iterating through it. See http://stackoverflow.com/questions/4081217/how-to-modify-list-entries-during-for-loop. — albert, Jun 06 '16 at 09:58

score 0 · Accepted Answer · edited Jun 06 '16 at 10:03

0

You are modifying the list you are iterating on. Iterate over a copy of x:

x=["foo_a","foo_a_b","foo"]
for i in list(x):
    if (("a" not in i) or ("b" in i)):
        x.remove(i)

Or even better, avoid using a for loop and create a new list with a list comprehension and assign it to x

x = ["foo_a","foo_a_b","foo"]
x = [item for item in x if not (("a" not in i) or ("b" in i))]

edited Jun 06 '16 at 10:03

zondo

answered Jun 06 '16 at 09:58

Andriy Ivaneyko

1 Answers1