Is file object in python an iterable

Question

I have a file "test.txt":

this is 1st line
this is 2nd line
this is 3rd line

the following code

lines = open("test.txt", 'r')
for line in lines:
    print "loop 1:"+line
for line in lines:
    print "loop 2:"+line

only prints:

loop 1:this is 1st line

loop 1:this is 2nd line

loop 1:this is 3rd line

It doesn't print loop2 at all.

Two questions:

the file object returned by open(), is it an iterable? that's why it can be used in a for loop?
why loop2 doesn't get printed at all?

score 54 · Accepted Answer · answered Jun 07 '13 at 23:57

54

It is not only an iterable, it is an iterator, which is why it can only traverse the file once. You may reset the file cursor with .seek(0) as many have suggested but you should, in most cases, only iterate a file once.

answered Jun 07 '13 at 23:57

jamylak

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+1 for a nice, concise description of the iterable-vs.-iterator thing (which most of the other answers didn't even attempt, even though it's the key to the question). – abarnert Jun 08 '13 at 00:03
1

Thanks for that description. – martinbshp Dec 29 '16 at 09:14

Martijn Pieters · Answer 2 · 2013-06-08T00:03:42.443

Yes, file objects are iterators.

Like all iterators, you can only loop over them once, after which the iterator is exhausted. Your file read pointer is at the end of the file. Re-open the file, or use .seek(0) to rewind the file pointer if you need to loop again.

Alternatively, try to avoid looping over a file twice; extract what you need into another datastructure (list, dictionary, set, heap, etc.) during the first loop.

score 3 · Answer 3 · answered Jun 07 '13 at 23:57

3

Yes, file objects are iterables but to go back to the beginning of the file you need to use lines.seek(0), since after the first loop you are at the end of the file.

answered Jun 07 '13 at 23:57

SethMMorton

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Mike Müller · Answer 4 · 2013-06-08T00:04:38.733

1

Your are already at the end of the file. File objects are iterators. Once you iterate over them your are at the final position. Iterating again won't start from the beginning. If you would like to start at line one again, you need to use lines.seek(0).

edited Jun 08 '13 at 00:04

answered Jun 07 '13 at 23:56

Mike Müller

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This is misleading. `list`s are also iterables, and yet you can iterate over them again without a `seek` or equivalent. The issue is that file objects are _iterators_ (as well as being iterables). – abarnert Jun 08 '13 at 00:04
Typo. I meant iterator. Thanks. Fixed. – Mike Müller Jun 08 '13 at 00:05

score 0 · Answer 5 · answered Jun 08 '13 at 00:00

0

It would be better, though, to rewrite code so that the file doesn't need to be iterated twice. Read all the lines into a list of some kind, or perform all the processing in a single loop.

answered Jun 08 '13 at 00:00

Andy G

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Is file object in python an iterable

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