gistfile1.py

def counter(start, interval):
    count = start
    while True:
        yield count
        count += interval

count = counter(start=1, interval=1)
def autoIncrement():
    return str(count.next()).zfill(3)

gistfile2.txt

>> print autoIncrement()
001
>> print autoIncrement()
002
>> print autoIncrement()
003
>> print autoIncrement()
004
>> print autoIncrement()
005
>> print autoIncrement()
006
>> print autoIncrement()
007