Writing a list to a file with Python, with newlines

Use a loop:

with open('your_file.txt', 'w') as f:
    for line in lines:
        f.write(f"{line}\n")

For Python <3.6:

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

For Python 2, one may also use:

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

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.

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)

Simpler is:

with open("outfile", "w") as outfile:
    outfile.write("\n".join(itemlist))

To ensure that all items in the item list are strings, use a generator expression:

with open("outfile", "w") as outfile:
    outfile.write("\n".join(str(item) for item in itemlist))

Remember that itemlist takes up memory, so take care about the memory consumption.

Yet another way. Serialize to json using simplejson (included as json in python 2.6):

>>> import simplejson
>>> f = open('output.txt', 'w')
>>> simplejson.dump([1,2,3,4], f)
>>> f.close()

If you examine output.txt:

[1, 2, 3, 4]

This is useful because the syntax is pythonic, it's human readable, and it can be read by other programs in other languages.