Convert Multiple Python Lists to CSV Columns

Question

My goal is to write a series of lists to a csv, where each list is a column.

This is my code so far:

rows = list(zip(columns['phase'],columns['start_date']))

with open('date_update.csv','w') as f:
    writer = csv.writer(f)
    for row in rows:
        writer.writerows(row);

This is the csv it produces. Both lists are the same length. When I print rows it has an extra '),' at the beginning. But this is not present when you print the two lists (columns['phase'] and columns['start_date']).

Is there something wrong with the zip line? Or am I not writing it to the csv file correctly?

Please don't post images. Help guidelines prefer text as it is searchable and cut-n-pastable. — Mark Tolonen
– Mark Tolonen, Commented Feb 21, 2019 at 2:01

Florian Toqué · Accepted Answer · 2019-02-21 00:24:33Z

1

You can use a pd.DataFrame and then save it to a csv. For example:

import pandas as pd
columns = {}

columns['phase'] = ['Phase 1','Phase 2', 'Phase 3','Phase 4']
columns['start_date'] = ['August 24 September','August 24 September',
                         'August 24 September', 'August 24 September']

data = list(zip(columns['phase'],columns['start_date']))

df = pd.DataFrame(data = data)

df.to_csv('date_update.csv', index=False, header=False)

Will save in the file 'date_update.csv' the following data:

Phase 1,August 24 September
Phase 2,August 24 September
Phase 3,August 24 September
Phase 4,August 24 September

edited Feb 21, 2019 at 0:24

answered Feb 21, 2019 at 0:13

Florian Toqué

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Mark Tolonen · Accepted Answer · 2019-02-21 02:14:07Z

Either use:

for row in rows:
    writer.writerow(row)

Or:

writer.writerows(rows)

Note the use of singular and plural forms. writerows requires a sequence of sequences. A string is a sequence, so passing a list of strings makes it use a string as a sequence of columns and list each character in a separate column.

Also per the csv docs, use either of the following to open the file passed to csv.writer:

with open('date_update.csv','wb') as f:           # Python 2
with open('date_update.csv','w',newline='') as f: # Python 3

Full example:

#!python3
import csv

columns = {}
columns['phase']      = ['Phase 1','Phase 2', 'Phase 3','Phase 4']
columns['start_date'] = ['Aug 24, 2018','Sep 25, 2018','Jan 01, 2019','Feb 15, 2019']

rows = zip(columns['phase'],columns['start_date'])

with open('date_update.csv','w',newline='') as f:
    writer = csv.writer(f)
    writer.writerows(rows)

Output:

Phase 1,"Aug 24, 2018"
Phase 2,"Sep 25, 2018"
Phase 3,"Jan 01, 2019"
Phase 4,"Feb 15, 2019"

Dan Temkin · Accepted Answer · 2019-02-21 02:34:42Z

0

As an addendum on what @mark-tolonen wrote you could also use DictWriter from the builtin csv module.

with open('date_update.csv','a') as f:
    writr = csv.DictWriter(f, fieldnames=["phase","start_date"])
    # writr.write_header() # Use only if file is created by operation or is empty
    writr.writerows([{"phase": c["phase"], "start_date": c["start_date"]} for c in columns])

answered Feb 21, 2019 at 2:34

Dan Temkin

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