Python 2.7 csv read, modify then write with dict? - python-2.7

Question

Ok I acknowledge that my question might duplicate this one but I have going to ask anyways 'cause although the ultimate goals are similar, the python code in use seems quite different.
I often have a list of students to create user accounts for. For this, I need to generate UserId's of the format
`Lastname[0:6].capitalize() + Firstname[0].capitalize()`
or six characters from the last name and First initial. I'd like to automate this with a python script reading from one .csv file containing firstname / lastname and writing firstname lastname userid to a different csv.
Here is my code, which almost works but I am having difficulty with the write rows to .csv part at the end:
import csv
input_file = csv.DictReader(open("cl.csv"))
index=0
fldnms=input_file.fieldnames
fldnms.append('UserName')
print fldnms
for row in input_file:
index+=1
UserID=(row["Last"][0:6].capitalize() + row["First"][0].capitalize())
row['UserName'] = UserID
print index, row["Last"], row["First"], row["UserName"]
with open("users.csv",'wb') as out_csv:
dw = csv.DictWriter(out_csv, delimiter=',', fieldnames=fldnms)
dw.writerow(dict((fn,fn) for fn in fldnms))
for row in input_file:
dw.writerow(row)
Advice / thoughts welcomed.
Thanks,
Brian H.

Answer 1

I went back to this after a good nights sleep and fwiw, here is the working version:
'''
Reads cl.csv (client list) as firstname lastname list with header
Writes users.csv as lastname firstname userid list w/o header
'''
import csv
INfile=open("..\cl_old.csv")
input_file = csv.DictReader(INfile, delimiter=' ')
fldnms={}
#print type (fldnms)
fldnms= input_file.fieldnames
fldnms.append('UserName')
#print type (fldnms)
#print (fldnms["Last"],fldnms["First"],fldnms["UserName"])
index =0
OUTfile=open("users.csv",'wb')
dw = csv.DictWriter(OUTfile, delimiter=',', fieldnames=fldnms)
dw.writerow(dict((fn,fn) for fn in fldnms))
for row in input_file:
index+=1
UserID=(row["Last"][0:6].capitalize() + row["First"][0].capitalize())
row['UserName'] = UserID
print index, row["Last"], row["First"], row["UserName"]
dw.writerow(row)
INfile.close()
OUTfile.close()
cl.csv contains a list of first, last name pairs. Results are stored in users.csv as names, userid.
I did this as an exercise in python as excel will do this in a single instruction.
=CONCATENATE(LEFT(A2,6),LEFT(B2,1))
Hope this is of interest.
BJH