How to loop twice over a list in Python

Question

I have a string containing CSV content. I turned it into a list like so:

from StringIO import StringIO
import csv

f = StringIO(list_csv)
my_list = csv.reader(f, delimiter=',')

Now I want to loop twice over the list, but the second time I try to loop, the code goes through as if the list was empty:

for item in my_list:
    code = item[1]
    print "code: "+code

for item in my_list:
    desc = item[2]
    print "desc: "+desc

(this code is simplified for demonstration purposes, I can't combine execution in a single loop)

I figured the list is still attached to the StringIO, and when it's done looping it's done reading, so it won't read again. Is there a one-liner to transform that into a regular list in the memory, that I could loop several times? I feel like running a loop manually copying values is reinventing the wheel.... but I can do that if there's no other option. Just asking for something less lengthy code-wise. Thanks!

my_list is a reader object, convert it to list, probably like this new_list = list(my_list) and loop over new_list — Sagun Shrestha
– Sagun Shrestha, Commented Dec 1, 2017 at 4:07
Thanks. You should've posted as an answer. ;) yeah.... that's how newb I am at python for now. :$ — msb
– msb, Commented Dec 2, 2017 at 1:06

Rick · Accepted Answer · 2017-12-01 04:19:59Z

7

The problem is that my_list is not actually a list. It is an iterator (actually a reader object in this particular case, which is just a type of iterator).

An iterator is consumed after the first time all the values have been yielded. This is what is happening to mylist: after the first time you loop through it, the reader object has already been consumed and cannot be iterated again.

One solution is to store the contents in an actual list object. This should fix it:

my_list = list(csv.reader(f, delimiter=','))

...then proceed as before.

By the way if you are using a recent version of Python 3, there's a really nice syntax available for unpacking the iterator to a list:

my_list = [*csv.reader(f, delimiter=',')]

edited Dec 1, 2017 at 4:19

answered Dec 1, 2017 at 4:08

Rick

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Radonirina Maminiaina · Accepted Answer · 2017-12-01 07:17:45Z

0

use this the following code:

my_list = list(csv.reader(f, delimiter=','))

edited Dec 1, 2017 at 7:17

Radonirina Maminiaina

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answered Dec 1, 2017 at 5:14

Nitin Prabhakar

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Menglong Li · Accepted Answer · 2017-12-01 07:22:41Z

0

Use tee function in itertools, which can provide you more accesses into the iterator, not just once.

from itertools import tee

i1, i2 = tee(my_list)

for i in i1:
    pass

for i in i2:
    pass

answered Dec 1, 2017 at 7:22

Menglong Li

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