How to call a function that is later in a Python script?

Question

I am currently learning Python for some penetration testing and was practicing making password cracking scripts. While I was making a script for a telnet pass cracker I ran into a problem with some of the functionality of it. While trying to allow the user to output the findings, as well as some extra information, I found my issue.

I am using getopt to take arguments for the script such as the ip, username, and an output file (I am trying to make an option to put in a word list for the passwords and usernames but I am still learning about using files). Because a function has to be written above where it is called I am running into the issue of needing the function in two places.

I need it above the getopt for loop, but I also need it in the for loop that guesses the password. I have looked at a few possible solutions but I am really confused by them as I am still somewhat new to Python. I do not really know how to explain it well but the basis of what I need to do is to be able to call the function before the function is written if anyone understands that. Thank you for all the help in advance.

Also I know that there are most likely a lot more efficient ways to do what I am trying, but I just wanted to mess around and see if I had the ability to do this, no matter how unorganized the code is.

Here is my code:

import telnetlib
import re
import sys
import time
import getopt
from time import gmtime, strftime

total_time_start = time.clock()

#Get the arguments from the user
try:
    opts, args = getopt.getopt(sys.argv[1:], "i:u:f:")
except getopt.GetoptError as err:
    print str(err)
    sys.exit(2)

passwords = ["hello","test", "msfadmin", "password"]
username = " "
ip = "0.0.0.0"
output_file = " "


for o, a in opts:
    if o == "-i":
        ip = a
    elif o in ("-u"):
        username =a 
    elif o in ("-f"):
        output_file = a
        file_out()
    else:
        assert False, "unhandled option"

#Connect using the password and username from the for loop later in the  script.
def connect(username, password, ip):
    global tn
    tn = telnetlib.Telnet(ip)

    print "[*] Trying " + username + " and " + password 

    tn.read_until("metasploitable login: ")
    tn.write(username + "\n")
    tn.read_until("Password: ")
    tn.write(password + "\n")

#Guess the password
for password in passwords:
    attempt = connect(username, password, ip)
    time_start = time.clock()
    if attempt == tn.read_until("msfadmin@metasploitable", timeout = 1):
        pass
    time_end = time.clock()
    time_finish = time_end - time_start
    #Determine if the password is correct or not
    if time_finish > 0.001000:
        print "\033[1;32;40m [*] Password '" + password + "' found for user '" + username+"'\033[0;37;40m\n"
        total_time_end = time.clock()

        total_time = (total_time_end - total_time_start)

        #Print the findings to a file that is selected from an argument
        def file_out():
            date = strftime("%a, %d %b %Y %H:%M:%S +0000", gmtime())
            fout = open(output_file, 'w')
            fout.write("Server IP: " + ip)
            fout.write("\nUsername is " + username)
            fout.write("Password is " + password)
            fout.write("\nCrack was conducted on " + date)
            fout.write("The crack took a total time of  " + total_time)

        sys.exit(0)

Here is the error I am getting:

python telnet_cracker.py -i [ip of metasploitable] -u msfadmin -f test.txt

Traceback (most recent call last):
  File "telnet_cracker.py", line 49, in <module>
    file_out()
NameError: name 'file_out' is not defined

Answer 1

Move the function to the top level of the script. Don't nest it inside of an if statement inside of a loop.

There's no need to redefine a function in a loop (and defining it in a conditional doesn't seem good either)

a function has to be written above where it is called

Not quite. Functions simply need to be defined before the code that runs them. The functions don't need to explicitly be "above the code" where they are called. Same logic applies to variables.

If you need to reference certain variables for the function, then use parameters.

Answer 2

Python is a dynamic language, but requires functions at the top-level to be resolved by interpretation time.

Just move the function to the top and call it later, or nest it within a function.

For example, this will not work:

x()

def x():
    pass

This however will work:

def x():
    pass
x()

And so will this:

def y():
    x()

def x():
    pass
y()

Python gives you all the tools to avoid forward declarations and circular dependencies with ease.