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.

I thought it would be interesting to explore the benefits of using a genexp, so here's my take.

The example in the question uses square brackets to create a temporary list, and so is equivalent to:

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

Which needlessly constructs a temporary list of all the lines that will be written out, this may consume significant amounts of memory depending on the size of your list and how verbose the output of str(item) is.

Drop the square brackets (equivalent to removing the wrapping list() call above) will instead pass a temporary generator to file.writelines():

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

This generator will create newline-terminated representation of your item objects on-demand (i.e. as they are written out). This is nice for a couple of reasons:

• Memory overheads are small, even for very large lists
• If str(item) is slow there's visible progress in the file as each item is processed

This avoids memory issues, such as:

In [1]: import os

In [2]: f = file(os.devnull, "w")

In [3]: %timeit f.writelines( "%s\n" % item for item in xrange(2**20) )
1 loops, best of 3: 385 ms per loop

In [4]: %timeit f.writelines( ["%s\n" % item for item in xrange(2**20)] )
ERROR: Internal Python error in the inspect module.
Below is the traceback from this internal error.

Traceback (most recent call last):
...
MemoryError

(I triggered this error by limiting Python's max. virtual memory to ~100MB with ulimit -v 102400).

Putting memory usage to one side, this method isn't actually any faster than the original:

In [4]: %timeit f.writelines( "%s\n" % item for item in xrange(2**20) )
1 loops, best of 3: 370 ms per loop

In [5]: %timeit f.writelines( ["%s\n" % item for item in xrange(2**20)] )
1 loops, best of 3: 360 ms per loop

(Python 2.6.2 on Linux)

In python>3 you can use print and * for argument unpacking:

with open("fout.txt", "w") as fout:
    print(*my_list, sep="\n", file=fout)

Simply:

with open("text.txt", 'w') as file:
    file.write('\n'.join(yourList))

file.write('\n'.join(list))

Using numpy.savetxt is also an option:

import numpy as np

np.savetxt('list.txt', list, delimiter="\n", fmt="%s")

I recently found Path to be useful. Helps me get around having to with open('file') as f and then writing to the file. Hope this becomes useful to someone :).

from pathlib import Path
import json
a = [[1,2,3],[4,5,6]]
# write
Path("file.json").write_text(json.dumps(a))
# read
json.loads(Path("file.json").read_text())

This logic will first convert the items in list to string(str). Sometimes the list contains a tuple like

alist = [(i12,tiger), 
(113,lion)]

This logic will write to file each tuple in a new line. We can later use eval while loading each tuple when reading the file:

outfile = open('outfile.txt', 'w') # open a file in write mode
for item in list_to_persistence:    # iterate over the list items
   outfile.write(str(item) + '\n') # write to the file
outfile.close()   # close the file 

You can also go through following:

Example:

my_list=[1,2,3,4,5,"abc","def"]
with open('your_file.txt', 'w') as file:
    for item in my_list:
        file.write("%s\n" % item)

Output:

In your_file.txt items are saved like:

1

2

3

4

5

abc

def

Your script also saves as above.

Otherwise, you can use pickle

import pickle
my_list=[1,2,3,4,5,"abc","def"]
#to write
with open('your_file.txt', 'wb') as file:
    pickle.dump(my_list, file)
#to read
with open ('your_file.txt', 'rb') as file:
    Outlist = pickle.load(file)
print(Outlist)

Output: [1, 2, 3, 4, 5, 'abc', 'def']

It save dump the list same as a list when we load it we able to read.

Also by simplejson possible same as above output

import simplejson as sj
my_list=[1,2,3,4,5,"abc","def"]
#To write
with open('your_file.txt', 'w') as file:
    sj.dump(my_list, file)

#To save
with open('your_file.txt', 'r') as file:
    mlist=sj.load(file)
print(mlist)

Another way of iterating and adding newline:

for item in items:
    filewriter.write(f"{item}" + "\n")

In Python3 You Can use this loop

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

Redirecting stdout to a file might also be useful for this purpose:

from contextlib import redirect_stdout
with open('test.txt', 'w') as f:
  with redirect_stdout(f):
     for i in range(mylst.size):
        print(mylst[i])

i suggest this solution .

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

i think you are looking for an answer like this.

f = open('output.txt','w')
list = [3, 15.2123, 118.3432, 98.2276, 118.0043]
f.write('a= {:>3d}, b= {:>8.4f}, c= {:>8.4f}, d= {:>8.4f}, e= 
{:>8.4f}\n'.format(*list))
f.close()