Nov 10, 2018

Best D-Bus lib for python out of the box

These days D-Bus is used in lot of places, on both desktop machines where it started, and servers too, with e.g. systemd APIs all over the place.

Never liked bindings offered for it in python though, as it's an extra half-C lib to worry about, and one that doesn't work too well either (had a lot of problems with it in earlier pulseaudio-mixer-cli script iterations).

But with systemd and its sd-bus everywhere, there's no longer a need for such extra lib, as its API is very easy to use via ctypes, e.g.:

import os, ctypes as ct

class sd_bus(ct.Structure): pass
class sd_bus_error(ct.Structure):
  _fields_ = [ ('name', ct.c_char_p),
    ('message', ct.c_char_p), ('need_free', ct.c_int) ]
class sd_bus_msg(ct.Structure): pass
lib = ct.CDLL('')

def run(call, *args, sig=None, check=True):
  func = getattr(lib, call)
  if sig: func.argtypes = sig
  res = func(*args)
  if check and res < 0: raise OSError(-res, os.strerror(-res))
  return res

bus = ct.POINTER(sd_bus)()
run( 'sd_bus_open_system', ct.byref(bus),
  sig=[ct.POINTER(ct.POINTER(sd_bus))] )

error = sd_bus_error()
reply = ct.POINTER(sd_bus_msg)()
  run( 'sd_bus_call_method',
      ct.c_char_p, ct.c_char_p, ct.c_char_p, ct.c_char_p,
      ct.c_char_p ] )
  dump = ct.c_char_p()
  run( 'sd_bus_message_read', reply, b's', ct.byref(dump),
    sig=[ct.POINTER(sd_bus_msg), ct.c_char_p, ct.POINTER(ct.c_char_p)] )
  return dump.value.decode()
finally: run('sd_bus_flush_close_unref', bus, check=False)

Which can be a bit more verbose than dbus-python API, but only needs libsystemd, which is already there on any modern systemd-enabled machine.

It's surprisingly easy to put/parse any amount of arrays, maps or any kind of variant data there, as sd-bus has convention of how each one is serialized to a flat list of values.

For instance, "as" (array of strings) would expect an int count with corresponding number of strings following it, or just NULL for empty array, with no complicated structs or interfaces.

Same thing for maps and variants, where former just have keys/values in a row after element count, and latter is a type string (e.g. "s") followed by value.

Example of using a more complicated desktop notification interface, which has all of the above stuff can be found here:

Whole function is a libsystemd-only drop-in replacement for a bunch of crappy modules which provide that on top of dbus.

sd-bus api is relatively new, but really happy that it exists in systemd now, given how systemd itself uses dbus all over the place.

Few script examples using it:

For small system scripts in particular, not needing to install any deps except python/systemd (which are always there), is definitely quite a good thing.

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