Querying Connectivity

You're application almost always needs to know if there is a working network connection. This is typically handled by placing the connection attempt in a try...catch block. That works, but can be slow, and it means the UI can't really adapt to the level of current connectivity. A much better solution is to query the NetworkManager [used by every mainstream distribution] via the System D-Bus for the current connectivity. This method is used by many applications from GNOME's Evolution to Mozilla's Firefox - but it doesn't seem to get much press coverage. So here is a simple example to query connectivity via Python [assuming NetworkManager 0.9 or later]:

#!/usr/bin/env python
import dbus

NM_BUS_NAME       = 'org.freedesktop.NetworkManager'
NM_OBJECT_PATH    = '/org/freedesktop/NetworkManager'
NM_INTERFACE_NAME = 'org.freedesktop.NetworkManager'
NM_STATE_INDEX = { 0: 'Unknown',
                   10: 'Asleep', 
                   20: 'Disconnected',
                   30: 'Disconnecting',
                   40: 'Connecting',
                   50: 'Connected (Local)',
                   60: 'Connected (Site)',
                   70: 'Connected (Global)' }

if __name__ == "__main__":
    bus = dbus.SystemBus()
    manager   = bus.get_object(NM_BUS_NAME, NM_OBJECT_PATH)
    interface = dbus.Interface(manager, NM_INTERFACE_NAME)

    state = interface.state()
    if state in NM_STATE_INDEX:
        print('Current Network State: {0}'.format(NM_STATE_INDEX[state]))
    else:
        print('Network Manager state not recognized.')

FYI: if you search the interwebz for the NetworkManager API specification ... every search engine will send you to the wrong place; either just wrong or to the documentation of an older version of the API. The current API specification is here.