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Quick question on how to properly write data back into a CSV file using the python csv module. Currently i'm importing a file, pulling a column of dates and making a column of days_of_the_week using the datetime module. I want to then write out a new csv file (or overright the individual one) containing one original element and the new element. 

with open('new_dates.csv') as csvfile2: readCSV2 = csv.reader(csvfile2, delimiter=',') incoming = [] for row in readCSV2: readin = row[0] time = row[1] year, month, day = (int(x) for x in readin.split('-')) ans = datetime.date(year, month, day) wkday = ans.strftime("%A") incoming.append(wkday) incoming.append(time) with open('new_dates2.csv', 'w') as out_file: out_file.write('\n'.join(incoming))

Input files looks like this:

2017-03-02,09:25 2017-03-01,06:45 2017-02-28,23:49 2017-02-28,19:34

When using this code I end up with an output file that looks like this:

Friday 15:23 Friday 14:41 Friday 13:54 Friday 7:13

What I need is an output file that looks like this:

Friday,15:23 Friday,14:41 Friday,13:54 Friday,7:13

If I change the delimiter in out_file.write to a comma I just get one element of data per column, like this:

Friday 15:23 Friday 14:41 Friday 13:54 ....

Any thoughts would be appreciated. Thanks!

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    The problem might be this join operation: '\n'.join(incoming). Try replacing that \n with a space to get the desired output; for a real csv, use "," Commented Apr 26, 2017 at 15:21
  • using a space results all data in one long row, instead of two CSV columns. And as noted the use of a comma results in each value in a different field in a single row. Commented Apr 26, 2017 at 15:25
  • What does the input file look like? Commented Apr 26, 2017 at 15:31
  • print wkday print time Thursday 16:45 Wednesday 20:42 Monday 12:12 Saturday 07:08 Commented Apr 26, 2017 at 15:37
  • Please edit the question to show the input file that creates the output. Make it reproducible. Commented Apr 26, 2017 at 15:45

4 Answers 4

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Being somewhat unclear on what format you want, I've assumed you just want a single space between wkday and time. For a quick fix, instead of appending both wkday and time separately, as in your example, append them together:

... incoming.append('{} {}'.format(wkday,time)) ...

OR, build your incoming as a list of lists:

... incoming.append([wkday,time]) ...

and change your write to:

with open('new_dates2.csv', 'w') as out_file: out_file.write('\n'.join([' '.join(t) for t in incoming]))
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4 Comments

Using that method seems to yield the same results as replace the \n with a space. All the data goes into a single field.
The second option makes some even stranger results. All data is in a single column and each field is one character.
@Justin sorry, missed a chunk out - above should work now
Traceback (most recent call last): File "./analyze.py", line 32, in <module> out_file.write('\n'.join(' '.join(incoming))) TypeError: sequence item 0: expected string, list found
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It seems you want Friday in column 0 and the time in column 1, so you need to change your incoming to a list of lists. That means the append statement should look like this:

... incoming.append([wkday, time]) ...

Then, it is better to use the csv.writer to write back to the file. You can write the whole incoming in one go without worrying about formatting.

with open('new_dates2.csv', 'w') as out_file: writer = csv.writer(out_file) writer.writerows(incoming)
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Results in an error: Traceback (most recent call last): File "./analyze.py", line 35, in <module> out_file.write('\n'.join(incoming)) TypeError: sequence item 0: expected string, list found
@Justin Check the answer as it currently stands - I edited it.
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Basically your incoming array is a linear list. So, you should have been doing is something like following:

#your incoming array incoming = ['Friday', '15:23', 'Friday', '14:41', 'Friday', '13:54', 'Friday', '7:13'] #actual parsing of the array for correct output for i,j in zip(incoming[::2], incoming[1::2]): out_file.write(','.join((i,j))) out_file.write('\n')
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You don't really need the csv module for this. I'm guessing at the input, but from the description it looks like:

2017-03-02,09:25 2017-03-01,06:45 2017-02-28,23:49 2017-02-28,19:34

This will parse it and write it in a new format:

import datetime with open('new_dates.csv') as f1, open('new_dates2.csv','w') as f2: for line in f1: dt = datetime.datetime.strptime(line.strip(),'%Y-%m-%d,%H:%M') f2.write(dt.strftime('%A,%H:%M\n'))

Output file:

Thursday,09:25 Wednesday,06:45 Tuesday,23:49 Tuesday,19:34
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This is assuming there's nothing else in the input file. OP seems to want CSV output too.
@Stuart The question changed since I wrote this, but still, adding a comma is a slight modification to the strftime. No assuming involved, since the OP also added the input.

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