Write lists of different size to csv in columns in Python

Question

I need to write lists that all differ in length to a CSV file in columns.

I currently have:

d=lists
writer = csv.writer(fl)
for values in zip(*d):
    writer.writerow(values)

which works only partially. What I suspect is happening is that it stops zipping up until the lists with the list of the shortest length.

Your issue is with how you are using zip. Check out this answer: http://stackoverflow.com/questions/19686533/how-to-zip-two-different-size-list-in-python — Krets, Jul 30 '14 at 18:25
@Krets I'm not sure that's the same question, though; the solutions there repeat the lists (like in `R`). — Al.Sal, Jul 30 '14 at 18:29
@Al.Sal You are correct. The answer I linked to relates to only two lists whereas this is for a list of lists. Regardless, the issue remains zip() will return a list that matches the length of the shortest input list. Sameer must manipulate all the lists to be the same length before using zip. The first answer seems like it would get it done. — Krets, Jul 30 '14 at 18:42
Hm. Then why not transpose the lists first? That would clean up code readibility-wise. — Al.Sal, Jul 30 '14 at 18:49

score 10 · Accepted Answer · answered Jul 30 '14 at 18:58

10

Code below is for Python 3. If you use Python 2, import izip_longest instead of zip_longest.

import csv
from itertools import zip_longest

d = [[2,3,4,8],[5,6]]

with open("file.csv","w+") as f:
    writer = csv.writer(f)
    for values in zip_longest(*d):
        writer.writerow(values)

Result:

2,5
3,6
4,
8,

answered Jul 30 '14 at 18:58

Christer Nissen

487
2
6

1

This created an empty row between every row – Stack_Protégé Mar 04 '21 at 22:10

Mark Whitfield · Answer 2 · 2014-07-30T18:37:12.180

There's no 'neat' way to write it since, as you said, zip truncates to the length of the shortest iterable.

Probably the simplest way would be to just pad with None or empty strings (Not sure offhand what the behavior of writerow is with None values):

maxlen = max([len(member) for member in d])
[member.extend([""] * (maxlen - len(member))) for member in d]
for values in zip(*d):
    ...

Alternatively, you could just construct each row inside the for loop instead of using zip, which would be more efficient but a lot wordier.

EDIT: corrected example

Write lists of different size to csv in columns in Python

2 Answers2

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