Python - List comprehension within File I/O code

Question

I want to replicate the functionality of the following code using a list comprehension:

with open('file.txt', 'w') as textfile:
    for i in range(1, 6):
        textfile.write(str(i) + '\n')

I tried the following:

with open('file.txt', 'w') as textfile:
    textfile.write(str([i for i in range(1, 6)]) + '\n')

but it (understandably) prints [1, 2, 3, 4, 5], instead of one number on a single line.

I don't have an answer to 'Why would you want to do that?'; I just want to see if it's possible. Thanks!

EDIT: Thank you all for the replies; for some reason I was under the impression that list comprehensions are always encapsulated in [].

list comprehensions are always surrounded by []. generator expressions are surrounded by () and can also be passed directly to functions without doubling the (()) — John La Rooy
– John La Rooy, Commented Feb 22, 2013 at 4:25
@gnibbler: Heard of generator expressions for the first time today. Thanks! — prgrmr
– prgrmr, Commented Feb 22, 2013 at 18:00

Gareth Latty · Accepted Answer · 2013-02-22 02:01:48Z

9

One way of doing this is file.writelines():

with open('file.txt', 'w') as textfile:
    textfile.writelines(str(i) + "\n" for i in range(1, 6))

answered Feb 22, 2013 at 2:01

Gareth Latty

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3 Comments

mgilson Over a year ago

Almost -- I don't think that writelines actually appends a newline.

Gareth Latty Over a year ago

@mgilson Indeed, updated - I was searching for the docs to check.

mgilson Over a year ago

writelines was my first impression too -- I don't know why I opted for join instead -- Maybe to keep as much of OP's original code as possible. Anyway, writelines is definitely cleaner here. +1

mgilson · Accepted Answer · 2013-02-22 02:01:14Z

2

textfile.write('\n'.join(str(i) for i in range(1,6)))

Is almost the same thing. This will leave off a trailing newline. If you need that, you could do:

textfile.write(''.join('{}\n'.format(i) for i in range(1,6)))

answered Feb 22, 2013 at 2:01

mgilson

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2 Comments

Arcturus Over a year ago

with open('/tmp/foo.txt','w') as f: [f.write(str(i)+' ') for i in range(0,10)]

mgilson Over a year ago

@Arcturus -- I don't actually recommend this. List-Comprehensions are for building lists. If you're not actually going to use the list that it builds, then I think is cleaner to just use a loop.

monkut · Accepted Answer · 2013-02-22 02:18:10Z

1

Also, if you're writing text that will be consumable by some other application, more often than not you're writing CSV, and the csv module makes these things easy.

(In this case you only have a single value per line, so this may not be needed.)

import csv

with open("file.txt", "wb") as out_f:
    writer = csv.writer(out_f)
    writer.writerows([[i] for i in range(1, 6)])

NOTE The csv module will take care of converting int to str for you.

answered Feb 22, 2013 at 2:18

monkut

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Comments

Arcturus · Accepted Answer · 2013-02-22 03:45:49Z

0

You could also try,

with open('/tmp/foo.txt','w') as f:
    [f.write(str(i)+' ') for i in range(0,10)]

with the examples of @monkut or @mgilson

answered Feb 22, 2013 at 3:45

Arcturus

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S. Lee · Accepted Answer · 2019-06-01 01:15:43Z

0

Please try the following:

open('/tmp/foo.txt','w').writelines(f'{i}\n' for i in range(10))

answered Jun 1, 2019 at 1:15

S. Lee

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