Write data from one csv to another python

It might be more efficient to read each input .csv into a dictionary before creating your aggregated result.

Here's a solution for reading in each file and storing the columns in a dictionary with Product_IDs as the keys. I assume that each Product_ID value exists in each file and that headers are included. I also assume that there are no duplicate columns across the files aside from Product_ID.

import csv
from collections import defaultdict

entries = defaultdict(list)
files = ['names.csv', 'costs.csv', 'descriptions.csv']
headers = ['Product_ID']

for filename in files:
   with open(filename, 'rU') as f:      # Open each file in files.
      reader = csv.reader(f)            # Create a reader to iterate csv lines
      heads = next(reader)              # Grab first line (headers)

      pk = heads.index(headers[0])      # Get the position of 'Product_ID' in
                                        # the list of headers
      # Add the rest of the headers to the list of collected columns (skip 'Product_ID')
      headers.extend([x for i,x in enumerate(heads) if i != pk])

      for row in reader:
         # For each line, add new values (except 'Product_ID') to the
         # entries dict with the line's Product_ID value as the key
         entries[row[pk]].extend([x for i,x in enumerate(row) if i != pk])

writer = csv.writer(open('result.csv', 'wb'))    # Open file to write csv lines
writer.writerow(headers)                         # Write the headers first
for key, value in entries.items():
   writer.writerow([key] + value)      # Write the product IDs
   # concatenated with the other values

A general solution that produces a record, maybe incomplete, for each id it encounters processing the 3 files needs the use of a specialized data structure that fortunately is just a list, with a preassigned number of slots

d = {id:[name,None,None] for id, name in [line.strip().split(',') for line in open(fn1)]}
for line in open(fn2):
    id, cost = line.strip().split(',')
    if id in d:
        d[id][1] = cost
    else:
        d[id] = [None, cost, None]
for line in open(fn3):
    id, desc = line.strip().split(',')
    if id in d:
        d[id][2] = desc
    else:
        d[id] = [None, None, desc]

for id in d:
    if all(d[id]): 
       print ','.join([id]+d[id])
    else: # for this id you have not complete info,
          # so you have to decide on your own what you want, I have to
        pass

If you are sure that you don't want to further process incomplete records, the code above can be simplified

d = {id:[name] for id, name in [line.strip().split(',') for line in open(fn1)]}
for line in open(fn2):
    id, cost = line.strip().split(',')
    if id in d: d[id].append(name)
for line in open(fn3):
    id, desc = line.strip().split(',')
    if id in d: d[id].append(desc)

for id in d:
    if len(d[id])==3: print ','.join([id]+d[id])