Take input and store them in variables and use them to create the instance
class StackOverflowUser:
def __init__(self, name, userid, rep):
self.name = name
self.userid = userid
self.rep = rep
name = raw_input("Enter name: ")
userid = int(raw_input("Enter user id: "))
rep = int(raw_input("Enter rep: "))
dave = StackOverflowUser(name, userid, rep)