For Loop for n iterations-Python

Question

I have a simple For Loop in a python script:

for filename in filenames:
    outline= getinfo(filename)
    outfile.write(outline)

This For Loop is part of a larger script that extracts data from HTML pages. I have nearly 6GB of html pages and want to do some test runs before I try it on all of them.

I have searched but cant find a way to make my For Loop break after n iterations (lets say 100.)

Use a counter and break if that counter is equal to 100. – squiguy Jun 11 '13 at 17:16 — squiguy, Jun 11 '13 at 17:16

score 11 · Answer 1 · answered Jun 11 '13 at 17:17

11

for filename in filenames[:100]:
    outline= getinfo(filename)
    outfile.write(outline)

The list slice filenames[:100] will truncate the list of file names to just the first 100 elements.

answered Jun 11 '13 at 17:17

kqr

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For a general iterator (not necessarily a list): `for filename in itertools.islice(filenames, 100):`. – chepner Jun 11 '13 at 17:24
@chepner Is that necessary also in Python 3? – kqr Jan 18 '18 at 14:45
Yes; you can't use slice syntax with arbitrary iterators. – chepner Jan 18 '18 at 15:41

score 8 · Answer 2 · answered Jun 11 '13 at 17:16

8

Keep a counter for your for loop. When your counter reaches, 100, break

counter = 0
for filename in filenames:
    if counter == 100:
        break
    outline= getinfo(filename)
    outfile.write(outline)
    counter += 1

answered Jun 11 '13 at 17:16

waldol1

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The preferred way to keep a counter is to do `for (counter, filename) in enumerate(filenames)`. – kqr Jun 11 '13 at 17:19

score 2 · Answer 3 · answered Jun 11 '13 at 17:25

2

I like @kqr's answer, but just another approach to consider, instead of taking the first 100, you could take a random n many instead:

from random import sample
for filename in sample(filenames, 10):
    # pass

answered Jun 11 '13 at 17:25

Jon Clements

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I would argue this is the better solution as long as it doesn't have any terrible performance issues. – kqr Jun 11 '13 at 17:28
@kqr the main issue I would be worried about is reproduce-ability.... So maybe the compromise is to take 1 in n instead, which could be done nicely with slicing as shown in your answer... And still be more useful for testing... – Jon Clements Jun 11 '13 at 17:30
Yes, that's what I thought too, but discarded since it would probably have similar performance to taking a random sample. I didn't think about testability, but you are indeed correct. – kqr Jun 11 '13 at 17:34

score 1 · Answer 4 · answered May 27 '20 at 21:22

1

Use the built-in function enumerate(), available in both Python 2 and 3.

for idx,filename in enumerate(filenames):
    if idx == 100:
        break
    outline= getinfo(filename)
    outfile.write(outline)

Also look at this.

answered May 27 '20 at 21:22

Orkun Berk Yuzbasioglu

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For Loop for n iterations-Python

4 Answers4