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Pexpect module for Python 2.5

Today I was searching for a way to run a bash command, that needed superuser’s permissions, from a python program that I had made… The problem was that I had to find a way to interact with the bash and when I would be asked for the password, the python program would answer with it.

So after some googling I found that awesome module called Pexpect , tested in Python 2.5, that allows to your python program to control and automate other programs.

As mentioned in its official site, Pexpect is basically a pattern matching system. It runs programs and watches output. When output matches a given pattern Pexpect can respond as if a human were typing responses. So it is very useful to automate some procedures, and others like testing and screen scraping. Also Pexpect is pure Python so it works on any platform that supports the standard Python pty module.

But enough with that!
Here is the code that I was talking about:

# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.

# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
# Author         : Aravanis Konstantinos
# Author's url   : http://AravanisKostas.com
# Author's email : kos.arav@gmail.com
#
# Tested in Python 2.5.5!

import pexpect
import getpass

class suError(Exception):
    """ A new exception. It is used from the su function, if there is a
        problem.
    """
    def __init__(self, value):
        self.value = value

    def __str__(self):
        return repr(self.value)

def su(command, password = None, prompt = "Enter SuperUser Password: ",\
        attempts = None):
    """ Function that ran a command with superuser's permissions.

        @command  : the command that must be ran with superuser's permissions
        @password : the superuser's password.
        @prompt   : the message that will be printed to inform that will ask
                    for the password, if it wasn't set.
        @attempts : the number of attempts. If it is not set then the program
                    will attempt for ever to gain the superuser's permissions.
                    If the attempts were set to zero, the program will still
                    run for a time.
    """
    # decrease the number of the remaining attempts
    if attempts:
        attempts = attempts - 1

    # if the password hadn't been given then ask for it (from the
    # command line)
    if not password:
        password = getpass.getpass(prompt)

    # the command must be ran with superuser's permissions
    child = pexpect.spawn('su -c "' + command + '"')

    # when the password is asked, send it
    choice = child.expect(['Password:'])
    child.sendline(password)

    # check to see if there was an Authentication failure or else that
    # there was no problem
    choice = child.expect(['su: Authentication failure' ,pexpect.EOF])

    # if there was an Authentication failure, ask again for the password
    if choice == 0:
        # if the max number of attempts was reached
        if attempts < 1:
            raise suError("Too Many Attempts!")

        su(command, attempts = attempts)

    # return success
    return True

And if you want to test it:

############
# test it! #
############
try:
    su("echo su_test_text > su_test_file", attempts = 3)
except suError:
    print "No other attempts left!"
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