Python script call from php not executing properly

Question

I have made a python script for getting a directory tree. Now I am trying to call this script from php using system().

My Python Script is:

import os
from sys import argv

script, start = argv

def list_files(startpath):

    directory = []

    for root, dirs, files in os.walk(startpath, topdown=True):

        for named in dirs:
            directory.append(os.path.join(root, named))

        for namef in files:
            directory.append(os.path.join(root, namef))

        return directory

dir = list_files(start)

My PHP command is:

$var = system('C:\\Python27\\python.exe generate -tree.py F:\\IET', $retval);
echo $var;
echo $retval;

The output that I am getting on the browser is the number '2', and when I was doing it a day before the output was coming as the number '1'.

I am new to PHP so couldn't really understand what I am doing wrong here. Any Guidance would be appreciated!