Python Dbus : How to export Interface property

Question

Hello,

In all python dbus documentations there are info on how to export objects, interfaces, signals, but there is nothing how to export interface property.

Any ideas how to do that ?

Regards, Levon

Answer 1

It's definitely possible to implement D-Bus properties in Python! D-Bus properties are just methods on a particular interface, namely org.freedesktop.DBus.Properties. The interface is defined in the D-Bus specification; you can implement it on your class just like you implement any other D-Bus interface:

# Untested, just off the top of my head

import dbus

MY_INTERFACE = 'com.example.Foo'

class Foo(dbus.service.object):
    # …

    @dbus.service.method(interface=dbus.PROPERTIES_IFACE,
                         in_signature='ss', out_signature='v')
    def Get(self, interface_name, property_name):
        return self.GetAll(interface_name)[property_name]

    @dbus.service.method(interface=dbus.PROPERTIES_IFACE,
                         in_signature='s', out_signature='a{sv}')
    def GetAll(self, interface_name):
        if interface_name == MY_INTERFACE:
            return { 'Blah': self.blah,
                     # …
                   }
        else:
            raise dbus.exceptions.DBusException(
                'com.example.UnknownInterface',
                'The Foo object does not implement the %s interface'
                    % interface_name)

    @dbus.service.method(interface=dbus.PROPERTIES_IFACE,
                         in_signature='ssv'):
    def Set(self, interface_name, property_name, new_value):
        # validate the property name and value, update internal state…
        self.PropertiesChanged(interface_name,
            { property_name: new_value }, [])

    @dbus.service.signal(interface=dbus.PROPERTIES_IFACE,
                         signature='sa{sv}as')
    def PropertiesChanged(self, interface_name, changed_properties,
                          invalidated_properties):
        pass

dbus-python ought to make it easier to implement properties, but it currently is very lightly maintained at best.

If someone fancied diving in and helping fix up stuff like this, they'd be most welcome. Even adding an expanded version of this boilerplate to the documentation would be a start, as this is quite a frequently asked question. If you're interested, patches could be sent to the D-Bus mailing list, or attached to bugs filed against dbus-python on the FreeDesktop bugtracker.