problem printing mount point of usb in python

Question

I have a program written in python that uses dbus to detect inserted usb drives and prints the dir they are mounted on when they are detected. Here is the code:

 
import dbus
import gobject
import shutil
import os
import subprocess
import time

class DeviceAddedListener:
    def __init__(self):
    self.bus = dbus.SystemBus()
        self.hal_manager_obj = self.bus.get_object(
                                              "org.freedesktop.Hal", 
                                              "/org/freedesktop/Hal/Manager")
        self.hal_manager = dbus.Interface(self.hal_manager_obj,
                                          "org.freedesktop.Hal.Manager")
    self.hal_manager.connect_to_signal("DeviceAdded", self._filter)

    def _filter(self, udi):
        device_obj = self.bus.get_object ("org.freedesktop.Hal", udi)
        device = dbus.Interface(device_obj, "org.freedesktop.Hal.Device")

        if device.QueryCapability("volume"):
            return self.do_something(device)

    def do_something(self, volume):
        device_file = volume.GetProperty("block.device")
        label = volume.GetProperty("volume.label")
        fstype = volume.GetProperty("volume.fstype")
        mounted = volume.GetProperty("volume.is_mounted")
        mount_point = volume.GetProperty("volume.mount_point")
        try:
            size = volume.GetProperty("volume.size")
        except:
            size = 0
    p1 = subprocess.Popen(["df", "-h"], stdout=subprocess.PIPE)
    p2 = subprocess.Popen(["grep", device_file], stdin=p1.stdout, stdout=subprocess.PIPE)
    p3 = subprocess.Popen(["awk", "{ print $6 }"], stdin=p2.stdout, stdout=subprocess.PIPE)
    path = p3.communicate()[0]
    print path



if __name__ == '__main__':
    from dbus.mainloop.glib import DBusGMainLoop
    DBusGMainLoop(set_as_default=True)
    loop = gobject.MainLoop()
    DeviceAddedListener()
    loop.run()

The problem is that when I print the path variable (the mount point of the usb), it prints an empty string. However, when I execute these same commands (Popen(), etc) in the python interactive interpreter, it prints the path just fine (/media/03CB-604C). Why does this occur? Any edits / suggestions to my code would be much appreciated. Thanks in advance!

Are the "p1... p2... lines indented wrong? Are they part of "do_something"? — mjhm
Allow me to make a suggestion -- doing this through dbus is far less efficient then if you did it through inotify (a kernel extension). If you have the option to build it in (or it'ds built in already), you can just watch the mount directory. pyinotify.sourceforge.net Also, there's no need to subprocess df/grep/awk -- what you want is in /proc/diskstats and can be extracted with a few lines of native python. — synthesizerpatel
@mjhm, yeah sorry about the poor indentation: in my actual code they are part of do_something, but when I transferred it to stack overflow the indentation got messed up. — kjakeb
@synthesizerpatel, I did not know about this kernel extension. I will try it out and see what happens! — kjakeb
@synthesizerpatel, ok well I haven't tried inotify yet but I did look at /proc/diskstats. In that file, can you tell me exactly how I can interpret that information and figure out the directory the usb is mounted on (remember, I am trying to manipulate the files on the usb) ? What I want to do is get this directory: /media/03CB-604, so that I can copy the files over from the usb to my machine. Thx! — kjakeb

ed. ed. · Accepted Answer · 2011-09-09T19:55:12

In your original question it seems likely that you're being beaten by a race condition.

The device is inserted and your code is executed before the mounting process has completed.

Try putting the Popen calls in a while loop (see below).

path = ""
count = 0
while count < 10 and path == "":
    p1 = subprocess.Popen(["df", "-h"], stdout=subprocess.PIPE)
    p2 = subprocess.Popen(["grep", device_file], stdin=p1.stdout, stdout=subprocess.PIPE)
    p3 = subprocess.Popen(["awk", "{ print $6 }"], stdin=p2.stdout, stdout=subprocess.PIPE)
    path = p3.communicate()[0]
    count += 1
    if path == "":
        time.sleep(1)
print path

This is a bit of a resource-hungry solution, but it should do what you want.

problem printing mount point of usb in python

1 Answers