Iterate thru directory of csvs in Python, find unique elements

iterate_thru_dic_csv_extract.py

#assume we have a bunch of files in a directory
#each file is csv... comma seperated. eg it has a bunch of lines like this"henry_1, 9939393, 02/11/1991, ARTS2bc1, ..."
#assume the first comma seperated value on each line in each file is the name
#Assume we want a list of unique names in all the files in the directory

import os

basedir="c://somedir//somesubdir//"

for root, dirs, files in os.walk(basedir): #iterates thru each file in directory, treating each file name as a string
    for file in files:
        thisfile=open(basedir+file, 'r')

        list1=thisfile.readlines()

        for i in range(len(list1)):
            #next line breaks the line into a list, splitting on commas
            list_items=list1[i].split(',')

            #we're assuming 'name' is always the first element in the comma seperated line
            for f in range(len(list_items)):
                 if f==0:
                      if list_names[f] not in list_names: #note: this can be slow... there are ways around this tho...
                           list_names.append(list_items[f])
        thisfile.close()


#finally, write your unique names to file
f_uniquenames=open('c://somedir//somesubdir//unique_names.txt', 'w')

for name in list_names:
    f_uniquenames.write(name+'\n')