Python Netcat Code, module main()

working on a netcat like program built in python (working through python programming book), I have all of the code written im just getting the following error when running "./netcat-page13.py -l -p 9999 -c"

Traceback (most recent call last):
  File "./netcat-page13.py", line 216, in <module>
    main()
  File "./netcat-page13.py", line 68, in main
    assert False,"Unhandled Option"
AssertionError: Unhandled Option

Code :

#!/usr/bin/python
import sys
import socket
import getopt
import threading
import subprocess

#define some global variables
listen              = False
command             = False
upload              = False
execute             = ""
target              = ""
upload_destination  = ""
port                = 0

def usage():
    print "BHP Net Tool"
    print
    print "Usage: bhpnet.py -t target_host -p port"
    print "-l --listen      -listen on [host]:[port] for incoming connections"
    print "-e --execute=file_to_run         -execute the given file upon receiving a connection"
    print "-c --command                 -initialize a command shell"
    print "-u --upload=destination           -Upon receiving connection upload a file and write to [destination]"
    print
    print
    print "Examples: "
    print "bhpnet.py -t 192.168.0.1 -p 5555 -l -c"
    print "bhpnet.py -t 192.168.0.1 -p 5555 -l -u=c:\\target.exe"
    print "bhpnet.py -t 192.168.0.1 -p 5555 -l -e=\"cat /etc/passwd\""
    print "echo 'ABCDEFGHI' | ./bhpnet.py -t 192.168.0.1 -p 135"
    sys.exit(0)

def main():
    global listen
    global port
    global execute
    global command
    global upload_destination
    global target

    if not len(sys.argv[1:]):
        usage()

    #read the commandline options
    try:
        opts, args = getopt.getopt(sys.argv[1:],"hle:t:p:cu:",["help","listen","execute","target","port","command","upload"])
    except getopt.GetoptError as err:
        print str(err)
        usage()

    for o,a in opts:
        if o in ("-h", "--help"):
            usage()
        elif o in ("-l", "--listen"):
            listen = True 
        elif o in ("e", "--execute"):
            execute = a
        elif o in ("c", "--commandshell"):
            command = True
        elif o in ("-u", "--upload"):
            upload_destination = a
        elif o in ("-t", "--target"):
            target = a
        elif o in ("p", "--port"):
            port = int(a)
        else:
            assert False,"Unhandled Option"

    #are we going to listen or just send data from stdin?
    if not listen and len(target) and port > 0:

        #read in the buffer from the commandline
        #this will block, so send CTRL-D if not sending input
        #to stdin
        buffer = sys.stdin.read()

        #send data off
        client_sender(buffer)

    #we are going to listen and potentially
    #upload things, execute commands, and drop a shell back
    #depending on our command line options above
    if listen:
        server_loop()

def client_sender(buffer):

    client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    try:
        #connect to our target host
        client.connect((target,port))

        if len(buffer):
            client.send(buffer)

        while True:

            #now wait for data back
            recv_len = 1
            response = ""

        while recv_len:

            data = client.recv(4096)
            recv_len = len(data)
            response+= data

            if recv_len < 4096:
                break

        print response,

        #wait for more input
        buffer = raw_input("")
        buffer += "\n"

        #send it off
        client.send(buffer)

    except:
        print "[*] Exception! Exiting"

        #tear down the connection
        client.close()

def server_loop():
    global target

    #if no target is defined, we listen on all interfaces
    if not len(target):
        target = "0.0.0.0"

    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    server.bind((target,port))
    server.listen(5)

    while True:
        client_socket, addr = server.accept()

        #spin off a thread to handle our new client
        client_thread = threading.Thread(target=client_handler, args=(client_Socket,))
        client_thread.start()

def run_command(command):

    #trim the newline
    command = command.rstrip()

    #run the command get the output back
    try:
        output = subprocess.check_ouput(command,stderr=subprocess.STDOUT, shell=True)
    except:
        output = "Failed to execute command.\r\n"

    #send the output back to the Client
    return output

def client_handler(client_socket):
    global upload
    global execute
    global command

    #check for upload
    if len(upload_destination):

        #read in all of the bytes and write to our destination
        file_buffer = ""

        #keep reading data until none is available
        while True:
            data = client_socket.recv(1024)

            if not data:
                break
            else:
                file_buffer += data

        #now we take these bytes and try to write them out
        try:
            file_descriptor = open(upload_destination,"wb")
            file_descriptor.write(file_buffer)
            file_descriptor.close()

            #acknowledge that we wrote the file out
            client_socket.send("Successfully saved file to %s\r\n" % upload_destination)
        except:
            client_socket.send("Failed to save file to %s\r\n" % upload_destination)

    #check for command execution
    if len(execute):

        #run the command
        output = run_command(execute)

        client_socket.send(output)

    #now we go into another loop if a command shell was requested
    if command:

        while True:
            #show a simple prompt
            client_socket.send("<BHP:#> ")

            #now we receive until we see a linefeed (enter key)
            cmd_buffer = ""
            while "\n" not in cmd_buffer:
                cmd_buffer += client_socket.recv(1024)

            #send back the command output
            response = run_command(cmd_buffer)

            #send back the response
            client_socket.send(response)
main()