import getpass
import telnetlib

HOST = "10.2.3.4"
user = input('Enter your username: ')
password = getpass.getpass()

#Telnet into the host
telnet  = telnetlib.Telnet(host) 

#Apply Username from input
telnet.read_until('Username: ')
telnet.write(user + '\n')

#Apply password if exists
if password:
    telnet.read_until('Password: ')
    telnet.write(password + '\r')

# Enter enable mode
telnet.write('enable\n')
telnet.write('password\n')

# Execute the Show Version command
telnet.write('show version\n')

# Exit the session
telnet.write('exit\n')

# Write output to screen
print(telnet.read_all())