Global Not Working Properly [Python]

Question

It's kind of hard to explain by situation. I'll try the best I can.

I'm making a program to make using driftnet easier for new users. You first enter the gateway IP and then the target IP, the your interface. In the program, once you enter all those, it opens a new terminal window, you then start the second program which flips the original order of the IPs. I could have it for the user to manually switch it, but I want it to just automatically switch it. To do that I have to use global to keep the input information, so it will just switch it. The problem is when I run the second program it just starts the first one all over again.

#This it the first program

#"Driftnet.py"


import os
import time
from subprocess import call

def drift():
    global gateway
    gateway = raw_input("Gateway IP > ")
    time.sleep(0.5)
    global target
    target = raw_input("Target IP > ")
    time.sleep(0.5)
    global inter
    inter = raw_input("Interface > ")
drift()

call(["gnome-terminal"])

os.system("arpspoof -i " + inter + " -t " + gateway + " " + target)

I run that, input everything, then it opens the second terminal, and I run the second program where it switches the IPs.

#This is the second program

#"Driftnet2.py"

import os
import time
from subprocess import call
import Driftnet

os.system("arpspoof -i " + Driftnet.inter + " -t " + Driftnet.target + " " + Driftnet.gateway)

When I run that it pretty much just runs the frist program, starting with the question of "Gateway IP > "

I have utterly no clue what I'm doing wrong.

Thanks

what happens if you print out `Driftnet.target` and `Driftnet.gateway` in the second program? — mitoRibo, Jul 21 '16 at 00:46

score 1 · Answer 1 · answered Jul 21 '16 at 00:54

I don't know what "Driftnet" is, and I'm not sure exactly what you're trying to do, but I can explain the behavior you're seeing.

In Driftnet2.py, you call:

import Driftnet

which causes the Python code in Driftnet.py to be evaluated. That is what the import statement does. All of your code is top-level (except the drift() method, which is called from the top-level), so importing it runs it.

You have only one method, and no top-level variables, so the global declarations are useless. (It almost seems like you think the global keyword is for IPC, but I'm not sure.)

To design this program, you need to first step back and answer some basic questions. Like, if you want to transfer information from Driftnet.py in one process to Driftnet2.py in another process, how is that transfer going to occur? (Command-line flags? Environmental variables? Unix domain sockets?) Once you know what you want your program to do, we can help you implement it.

You sound like you know what you're doing, too bad I'm kinda new to python. I don't really know exactly what I need to do, but what I want is to store the information from the raw_input variables in Driftnet.py and use that information in Driftnet2.py. I don't know the method I should use to do this, but what do you suggest? — Gray Noneya, Jul 21 '16 at 01:17
As I said, I don't know what "Driftnet" is or how it needs to be set up. If you want two programs to talk to each other, you'll need to learn about IPC, which is more than can be described in a comment here. — J. Cocoe, Jul 21 '16 at 16:03

score 0 · Answer 2 · answered Jul 21 '16 at 00:46

0

Have you tried adding the global variable after import etc...then calling the var inside functions as global?

import os
import time
from subprocess import call
gateway = ''
target = ''
inter = ''

answered Jul 21 '16 at 00:46

Shawon Md

195
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On the second or first program? – Gray Noneya Jul 21 '16 at 00:59
Try with first one. – Shawon Md Jul 21 '16 at 01:03
When I do this is says "Segmentation fault" and doesn't start using arpspoof, but _does_ open a new terminal – Gray Noneya Jul 21 '16 at 01:05
hmm.. it works in simple cases..Not sure whats this code is gonna do..If I had to do this I would run first py then call the second one in class fashion that way i can send the variables to the class easily. – Shawon Md Jul 21 '16 at 01:08

score 0 · Answer 3 · edited May 23 '17 at 12:08

This is what the if __name__=='__main__': construct exists for. Change the code in Driftnet.py to this:

import os
import time
from subprocess import call

def drift():
    global gateway
    gateway = raw_input("Gateway IP > ")
    time.sleep(0.5)
    global target
    target = raw_input("Target IP > ")
    time.sleep(0.5)
    global inter
    inter = raw_input("Interface > ")

if __name__=='__main__':
    drift()

    call(["gnome-terminal"])

    os.system("arpspoof -i " + inter + " -t " + gateway + " " + target)

It's possible that you also have modify Driftnet2.py a little. I'm not sure if you'll be able to access the global variables by doing import Driftnet. If you're getting errors, try import __main__ as Driftnet instead.

Global Not Working Properly [Python]

3 Answers3