python how to write list of lists to file

Question

I am trying to write a list of lists of strings to a file. My big list is of the forme:

 1.0 '0:25.0' '1:50.0' '2:131.0' '3:202.0'
 1.0 '0:2.0'  '1:36.0' '2:131.0' '3:188.0'
-1.0 '0:56.0' '1:53.0' '2:55.0'  '3:58.0'
-1.0 '0:50.0' '1:51.0' '2:48.0'  '3:55.0'  
 and so on ... 

the first column is of type int and the remaining are of type str.

I have tried the following code:

f = open('dataset', 'w')
for i in range(len(mylist[0]):
     f.write(str(item) for item in mylist[i])
f.close()

But I get the following error:

TypeError: expected a character buffer object

the error occured in line 3 of the above code (f.write(...))

EDIT:

I have tried:

 with open("train_data.csv", 'w') as f:
     writer = csv.writer(f)
     writer.writerows(listdata)

But, surprisingly, for lines starting with -1.0 I only get the 1st 31 elements ( each line (list) contains 362 elements). I have no idea why this is happening.

Answer 1

Use the csv library:

import  csv

with open("out.csv","w") as f:
    wr = csv.writer(f)
    wr.writerows(list_of_lists)

writerows takes an iterable of iterables so the content of each sublist will be written to each row.

So for list_of_lists = [[1,2,3],[4,5,6],[7,8,9]] your out.csv would look like:

1,2,3
4,5,6
7,8,9

And as Scironic commented, you can pass whatever delimiter you want, ie wr = csv.writer(f,delimiter=" ") for a space etc..

Answer 2

I would do like this.

from __future__ import with_statement

mylist = [[1.0,'0:25.0','1:50.0'],[-1.0,'0:56.0','1:53.0']] # as example

with open('output.txt', 'w') as f:
    for _list in mylist:
        for _string in _list:
            #f.seek(0)
            f.write(str(_string) + '\n')

As a result, I get this in output.txt:

1.0
0:25.0
1:50.0
-1.0
0:56.0
1:53.0