An element lost when I use 'for' in python, what happened?

Question

I wrote a program of python as below:

from os import listdir 
def getfilename():
    namelist=listdir(".")
    print namelist
    for name in namelist:
        print name
        if 'lp' in name:
            namelist.remove(name)
    return namelist

when I run it on the command line and print the return value, the result is :

['all_test1.list', 'all_test2.list', 'lp.py', 'lp.pyc', 'lp.test', 'straightrnd_test1.list', 'straightrnd_test2.list', 'straightrnd_train.list']
all_test1.list
all_test2.list
lp.py
lp.test
straightrnd_test2.list
straightrnd_train.list
['all_test1.list', 'all_test2.list', 'lp.pyc', 'straightrnd_test1.list', 'straightrnd_test2.list', 'straightrnd_train.list']

The file 'lp.pyc' lost in for loop, so the return value contains it, I wanna remove it from the list,what should I do? Why the element lost in for loop?

score 2 · Answer 1 · answered Mar 15 '14 at 12:12

2

You are altering the list you are iterating over. Use list comprehensions:

newlist = [x for x in namelist if 'lp' not in x]

answered Mar 15 '14 at 12:12

zero_dev

613
9
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score 1 · Accepted Answer · edited May 23 '17 at 12:21

1

This question addresses the problem pretty well but in a nutshell:

Even though you don't explicitely loop over the list via the index of the elements, Python sort-of does it for you internally.

It's really as if you wrote something like:

l = listdir(".")
while i < len(l):
    name = l[i]
    if 'lp' in name:
        l.remove(name)
        # Now evaluating l[i] yields another element, even though i was not incremented !

    i = i + 1

So when you remove an element during the iteration, it shifts the loop pointer one time too many, hence the problem you are observing.

Note that in your case, you could just write:

from os import listdir 

def getfilename():
    return [x for x in listdir(".") if not 'lp' in x]

Even though the name getfilename is a bit weird, given that it actually returns a list... but that's another issue.

edited May 23 '17 at 12:21

Community

1
1

answered Mar 15 '14 at 12:12

ereOn

53,676
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161
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what should I do to avoid this? – ChaosCosmos Mar 15 '14 at 12:14
1

@ChaosCosmos: Use the list-comprehension syntax instead. It's more Pythonic and expresses the intent in a much better way. – ereOn Mar 15 '14 at 12:17
Thank you very much, I know the grammar of lambda anonymous function, since it's too weird, I can't recall it when programming. – ChaosCosmos Mar 15 '14 at 12:22
@ChaosCosmos: You are welcome. Note that this is not a lambda, but a list-comprehension. lambdas in Python are a completely different beast. – ereOn Mar 15 '14 at 12:26
I mixed them up, thanks for your correction. – ChaosCosmos Mar 15 '14 at 12:31

An element lost when I use 'for' in python, what happened?

2 Answers2