How can I parse this list of comma separated values

Question

I have a list of comma-separated users and their permissions, where some users have up to 8 permissions:

user_a, permission_1
user_b, permission_1, permission_2
user_c, permission_1, permission_2, permission_3

I need my output to be a list of comma-separated users and permissions, where every user and permission pair are on a separate line:

user_a, permission_1
user_b, permission_1
user_b, permission_2
user_c, permission_1
user_c, permission_2
user_c, permission_3

My python code below using the csv module is currently not working:

import csv

with open('user_list.csv') as user_list:
    dict_reader = csv.DictReader(user_list)
    ul = []
    for row in dict_reader:
        ul.append(row)

with open('output.txt','w') as output:
    fields = 'USER','PERM1','PERM2','PERM3','PERM4',
    'PERM5','PERM6','PERM7','PERM8'
    dict_writer = csv.DictWriter(output, fields)
    csv.DictWriter.writeheader(dict_writer)
    for i in ul:
        dict_writer.writerow(i)

It's giving me the following Traceback:

Traceback (most recent call last):
  File "cops_ul_parse.py", line 15, in <module>
    dict_writer.writerow(i)
  File "C:\Python27\lib\csv.py", line 148, in writerow
    return self.writer.writerow(self._dict_to_list(rowdict))
  File "C:\Python27\lib\csv.py", line 144, in _dict_to_list
    ", ".join(wrong_fields))
ValueError: dict contains fields not in fieldnames: PERM8, PERM7, PERM6, PERM5

Any idea why this isn't working? Or, is there a better way to accomplish my desired output? Maybe a regex would work better?

Answer 1

The issue is this line:

fields = 'USER','PERM1','PERM2','PERM3','PERM4',
'PERM5','PERM6','PERM7','PERM8'

Your code is doing fields = 'USER','PERM1','PERM2','PERM3','PERM4', (remember tuples and lists can have trailing commas after the last item), and then making another tuple ( 'PERM5','PERM6','PERM7','PERM8' ) and doing nothing with it.

Then, when you make the csv.DictWriter , it only has the fields from the first tuple - hence the error you are getting.

The solution is simple: use brackets around your tuple, which then gives you implied line continuation:

fields = ('USER','PERM1','PERM2','PERM3','PERM4',
          'PERM5','PERM6','PERM7','PERM8')

You can use a backslash to escape the line break too, but brackets are much nicer to read.

Answer 2

for perm in listOfPermissions:
    user, perms = perm.split(',', 1)
    for p in perms:
        print "%s, %s" %(user, p)

To write this to a file:

with open('path/to/file', 'w') as outfile:
    for perm in listOfPermissions:
        user, perms = perm.split(',', 1)
        for p in perms:
            outfile.write("%s, %s\n" %(user, p))