Mpris2 interfaces don't implement introspection for properties

[FichteFoll]

It looks like the mpris2 module doesn't implement any properties. Via FichteFoll/discordrp-mpris#4, I was made aware that the PlaybackStatus property isn't implemented and a quick glance at the source seems to indicate that in fact no property is exposed at all. I'm not familiar with the Python dbus package, but a quick glance with QDBusViewer seems to confirm this.
(Assuming dbus provides the org.freedesktop.DBus.Introspectable automatically.)

[hrnr]

The properties are implemented, but they are not introspectable due to limitations of dbus-python we use to implement dbus interface. Even signals for properties should work as expected.

In your screenshot you see that there is org.freedesktop.DBus.Properties under /org/mpris/MediaPlayer2. That is a standard property interface. Eg.

GetAll("org.mpris.MediaPlayer2")

{'CanQuit': False,
 'CanRaise': False,
 'CanSetFullscreen': False,
 'DesktopEntry': 'mps-youtube',
 'Fullscreen': False,
 'HasTrackList': False,
 'Identity': 'mps-youtube',
 'SupportedMimeTypes': [],
 'SupportedUriSchemes': []}

GetAll("org.mpris.MediaPlayer2.Player")

{'CanControl': True,
 'CanGoNext': True,
 'CanGoPrevious': True,
 'CanPause': True,
 'CanPlay': True,
 'CanSeek': True,
 'MaximumRate': 1.0,
 'Metadata': {'mpris:artUrl': 'https://i.ytimg.com/vi/S3fTw_D3l10/default.jpg',
              'mpris:length': 307000000,
              'mpris:trackid': '/CurrentPlaylist/ytid/S3fTwD3l10',
              'xesam:album': '',
              'xesam:artist': [''],
              'xesam:title': 'Alt-J (∆) - Taro'},
 'MinimumRate': 1.0,
 'PlaybackStatus': 'Playing',
 'Position': 2987500,
 'Rate': 1.0,
 'Volume': 1.0}

For implementation see https://github.com/mps-youtube/mps-youtube/blob/0100a38d3874c53e705d9acd7e30fbc6a3099da0/mps_youtube/mpris.py#L462

[FichteFoll]

Ah right, I forgot that this is fundamentally how you access properties. For obious reasons, QDBusViewer uses introspection to show a bus's path's interfaces and the methods and properties they define, so it's understandable why I my method to "check" for them fell into the same trap as the library I use to access the properties in my tool (dbussy). Calling GetAll does indeed return the proper properties.

Judging from your reply I take that this is more of a problem of dbus-python because it doesn't provide an easy/proper way to declare introspectable properties?

[hrnr]

yes, dbus-python does not support properties first class as far as I know. However the only thing that is missing are the introspection data, which should not be required.

I'm not sure it is possible to add introspection data for properties in the current implementation, because dbus-python already generates introspection for methods and offers Introspectable interface on our bus.