I've been writing a basic script to enumerate SMTP users (via a user dictionary) on poorly configured SMTP servers. In scripts like this, I usually see arguments handled as follows:if (len(sys.argv) != 3:    print Usage: ...    sys.exit(0)I found the if and print statement approach irritating, but without a concrete reason why in my mind. I decided to use assertions and combine argument handling with general setup. The following is the complete script.#!/usr/bin/pythonimport socketimport sysimport osSMTP_PORT = 25## Helper Functions#def create_connection(host, port):    smtp_server = sys.argv[1]    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)    sock.connect((host, port))    return(sock)def smtp_verify(username, smtp_conn):    smtp_conn.send('VRFY ' + username + '\r\n')    smtp_reply = smtp_conn.recv(1024)    return(smtp_reply)## Arg checking, connection setup#try:    assert len(sys.argv) == 3, \    Usage: smtp_enum.py <smtp_server_ip> <username_list_file>    users_file = sys.argv[2]    assert os.path.isfile(users_file), Cannot open file %s % users_file    smtp_server = sys.argv[1]    smtp_conn = create_connection(smtp_server, SMTP_PORT)    banner = smtp_conn.recv(1024)    print bannerexcept Exception as e:    print Error: %s % e    exit(1)## Perform SMTP enumeration#with open(users_file, 'r') as users:    for user in users:        print smtp_verify(user.strip(), smtp_conn)smtp_conn.close()I'm wondering if I may later regret the assertion approach if the script grows. Is this an abuse of assertions, or somehow problematic?
Verifying user accounts on SMTP servers
python;error handling;email;assertions
What you really want is a robust argument parsing. This calls for argparse:import argparseimport os.pathdef is_file(f):    if os.path.isfile(f):        return f    raise OSError({} does not exist.format(f))parser = argparse.ArgumentParser(description=SMTP user checker)parser.add_argument('server', help=IP address of server)parser.add_argument('usernames', type=is_file, help=Path to list of user names)parser.add_argument('--port', '-p', type=int, default=25, help=SMTP port of server)args = parser.parse_args()smtp_conn = create_connection(args.server, args.port)print smtp_conn.recv(1024)Note that it is already standard practice of Python to print any error and exit afterwards.Additionally, you probably want to close your SMTP connection even in the case of an exception. For this, you could use a contextmanager:from contextlib import contextmanager@contextmanagerdef create_connection(host, port):    try:        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)        sock.connect((host, port))        yield sock    except socket.error:        # Handle specific exception here, or not and re-raise        raise    finally:        # Ensure sock is closed under all circumstances        try:            sock.close()        except UnboundLocalError:            # Did not even succeed in creating the variable, nothing left to do            passAnd then use it like this:with create_connection(args.server, args.port) as smtp_conn:    print smtp_conn.recv(1024)    with open(args.usernames) as users:        for user in users:            print smtp_verify(user.strip(), smtp_conn)Note that Python uses read mode by default when opening a file, so there is no need to use 'r'.Final code:import argparseimport socketimport os.pathfrom contextlib import contextmanager@contextmanagerdef create_connection(host, port):    try:        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)        sock.connect((host, port))        yield sock    except socket.error:        # Handle specific exception here, or not and re-raise        raise    finally:        # Ensure sock is closed under all circumstances        try:            sock.close()        except UnboundLocalError:            # Did not even succeed in creating the variable, nothing left to do            passdef smtp_verify(username, smtp_conn):    smtp_conn.send('VRFY {}\r\n'.format(username))    return smtp_conn.recv(1024)def is_file(f):    if os.path.isfile(f):        return f    raise OSError({} does not exist.format(f))def parse_args():    parser = argparse.ArgumentParser(description=SMTP user checker)    parser.add_argument('server', help=IP address of server)    parser.add_argument('usernames', type=is_file, help=Path to list of user names)    parser.add_argument('--port', '-p', type=int, default=25, help=SMTP port of server)    return parser.parse_args()if __name__ == __main__:    args = parse_args()    with create_connection(args.server, args.port) as smtp_conn:        print smtp_conn.recv(1024)        with open(args.usernames) as users:            for user in users:                print smtp_verify(user.strip(), smtp_conn)Alternatively, you could use the docopt module for this:SMTP user checker.Usage:  argparser_comp.py SERVER USERNAMES [(-p <p>| --port=<p>)]  argparser_comp.py (-h | --help)Options:  -h --help          Show this screen.  -p <p> --port=<p>  SMTP port of server [default: 25].import socketimport os.pathfrom contextlib import contextmanagerfrom docopt import docoptdef is_file(f):    if os.path.isfile(f):        return f    raise OSError({} does not exist.format(f))def parse_args():    args = docopt(__doc__)    return args[SERVER], is_file(args[USERNAMES]), int(args[--port])@contextmanagerdef create_connection(host, port):    try:        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)        sock.connect((host, port))        yield sock    except socket.error:        # Handle specific exception here, or not and re-raise        raise    finally:        # Ensure sock is closed under all circumstances        try:            sock.close()        except UnboundLocalError:            # Did not even succeed in creating the variable, nothing left to do            passdef smtp_verify(username, smtp_conn):    smtp_conn.send('VRFY {}\r\n'.format(username))    return smtp_conn.recv(1024)if __name__ == __main__:    server, usernames, port = parse_args()    with create_connection(server, port) as smtp_conn:        print smtp_conn.recv(1024)        with open(usernames) as users:            for user in users:                print smtp_verify(user.strip(), smtp_conn)