python - csv: writer.writerows() splitting my string inputs

Question

I have a list of strings which I would like to write to a csv file. The list list_results looks like

['False, 60, 40 ', 'True, 70, 30, ']

So, I would try something like this:

with open('example1.csv', 'w') as result:
    writer = csv.writer(result, delimiter=",")
    writer.writerow( ('Correct?', 'Successes', 'Failures') )
    writer.writerows(list_results)

Unfortunately, the data doesn't write into three columns. I find:

['Correct?', 'Successes', 'Failures']  
['F', 'a', 'l', 's', 'e', ',', ' ', '6', '0', ',', ' ', '4', '0', ' ']  
['T', 'r', 'u', 'e', ',', ' ', '7', '0', ',', ' ', '3', '0', ',', ' ']

How do I format this behavior appropriately to get

['Correct?', 'Successes', 'Failures']  
['False', 60, 40]  
['True', 70, 30]

Preferably in format %s %d %d ?

Answer 1

You are using writer.writerows() with an s at the end. That method expects a list of lists, but you passed in a list of strings. The writerows() method essentially does this:

def writerows(self, rows):
    for row in rows:
        self.writerow(row)

where each row must be a sequence of columns. A string is a sequence of individual characters, so that's what you get written: individual characters separated by your chosen delimiter.

You'll need to split out your string into columns, don't include the commas yourself, it's the job of the writer object to include those:

with open('example1.csv', 'w') as result:
    writer = csv.writer(result, delimiter=",")
    writer.writerow(('Correct?', 'Successes', 'Failures'))
    for row in list_results:
        columns = [c.strip() for c in row.strip(', ').split(',')]
        writer.writerow(columns)

or using a generator expression so you can keep using writerows():

with open('example1.csv', 'w') as result:
    writer = csv.writer(result, delimiter=",")
    writer.writerow(('Correct?', 'Successes', 'Failures'))
    writer.writerows([c.strip() for c in r.strip(', ').split(',')]
                     for r in list_results)

Demo:

>>> import csv
>>> list_results = ['False, 60, 40 ', 'True, 70, 30, ']
>>> import csv
>>> import sys
>>> list_results = ['False, 60, 40 ', 'True, 70, 30, ']
>>> writer = csv.writer(sys.stdout)
>>> writer.writerow(('Correct?', 'Successes', 'Failures'))
Correct?,Successes,Failures
>>> for row in list_results:
...     columns = [c.strip() for c in row.strip(', ').split(',')]
...     writer.writerow(columns)
...
False,60,40
True,70,30
>>> writer.writerows([c.strip() for c in r.strip(', ').split(',')]
...                  for r in list_results)
False,60,40
True,70,30