Writing a list to a file with Python


Question

Is this the cleanest way to write a list to a file, since writelines() doesn't insert newline characters?

file.writelines(["%s\n" % item  for item in list])

It seems like there would be a standard way...

1
555
1/16/2016 9:32:46 PM

Accepted Answer

You can use a loop:

with open('your_file.txt', 'w') as f:
    for item in my_list:
        f.write("%s\n" % item)

In Python 2, you can also use

with open('your_file.txt', 'w') as f:
    for item in my_list:
        print >> f, item

If you're keen on a single function call, at least remove the square brackets [], so that the strings to be printed get made one at a time (a genexp rather than a listcomp) -- no reason to take up all the memory required to materialize the whole list of strings.

768
5/14/2019 5:39:08 PM

What are you going to do with the file? Does this file exist for humans, or other programs with clear interoperability requirements?

If you are just trying to serialize a list to disk for later use by the same python app, you should be pickleing the list.

import pickle

with open('outfile', 'wb') as fp:
    pickle.dump(itemlist, fp)

To read it back:

with open ('outfile', 'rb') as fp:
    itemlist = pickle.load(fp)

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