# dbus-python tutorial

Author:

  Simon McVittie, Collabora Ltd.

Date:

  2006-06-14

This tutorial requires Python 2.4 or up, and `dbus-python` 0.80rc4 or up.

## Connecting to the Bus

Applications that use D-Bus typically connect to a bus daemon, which forwards messages between the applications. To use D-Bus, you need to create a `Bus` object representing the connection to the bus daemon.

There are generally two bus daemons you may be interested in. Each user login session should have a session bus, which is local to that session. It’s used to communicate between desktop applications. Connect to the session bus by creating a `SessionBus` object:

    import dbus

    session_bus = dbus.SessionBus()

The system bus is global and usually started during boot; it’s used to communicate with system services like udev, NetworkManager, and the Hardware Abstraction Layer daemon (hald). To connect to the system bus, create a `SystemBus` object:

    import dbus

    system_bus = dbus.SystemBus()

Of course, you can connect to both in the same application.

For special purposes, you might use a non-default Bus, or a connection which isn’t a Bus at all, using some new API added in dbus-python 0.81.0. This is not described here, and will at some stage be the subject of a separate tutorial.

## Making method calls

D-Bus applications can export objects for other applications’ use. To start working with an object in another application, you need to know:

- The bus name. This identifies which application you want to communicate with. You’ll usually identify applications by a well-known name, which is a dot-separated string starting with a reversed domain name, such as `org.freedesktop.NetworkManager` or `com.example.WordProcessor`.

- The object path. Applications can export many objects - for instance, example.com’s word processor might provide an object representing the word processor application itself and an object for each document window opened, or it might also provide an object for each paragraph within a document.

  To identify which one you want to interact with, you use an object path, a slash-separated string resembling a filename. For instance, example.com’s word processor might provide an object at `/` representing the word processor itself, and objects at `/documents/123` and `/documents/345` representing opened document windows.

As you’d expect, one of the main things you can do with remote objects is to call their methods. As in Python, methods may have parameters, and they may return one or more values.

### Proxy objects

To interact with a remote object, you use a proxy object. This is a Python object which acts as a proxy or “stand-in” for the remote object - when you call a method on a proxy object, this causes dbus-python to make a method call on the remote object, passing back any return values from the remote object’s method as the return values of the proxy method call.

To obtain a proxy object, call the `get_object` method on the `Bus`. For example, NetworkManager has the well-known name `org.freedesktop.NetworkManager` and exports an object whose object path is `/org/freedesktop/NetworkManager`, plus an object per network interface at object paths like `/org/freedesktop/NetworkManager/Devices/eth0`. You can get a proxy for the object representing eth0 like this:

    import dbus
    bus = dbus.SystemBus()
    proxy = bus.get_object('org.freedesktop.NetworkManager',
                           '/org/freedesktop/NetworkManager/Devices/eth0')
    # proxy is a dbus.proxies.ProxyObject

### Interfaces and methods

D-Bus uses interfaces to provide a namespacing mechanism for methods. An interface is a group of related methods and signals (more on signals later), identified by a name which is a series of dot-separated components starting with a reversed domain name. For instance, each NetworkManager object representing a network interface implements the interface `org.freedesktop.NetworkManager.Devices`, which has methods like `getProperties`.

To call a method, call the method of the same name on the proxy object, passing in the interface name via the `dbus_interface` keyword argument:

    import dbus
    bus = dbus.SystemBus()
    eth0 = bus.get_object('org.freedesktop.NetworkManager',
                          '/org/freedesktop/NetworkManager/Devices/eth0')
    props = eth0.getProperties(dbus_interface='org.freedesktop.NetworkManager.Devices')
    # props is a tuple of properties, the first of which is the object path

As a short cut, if you’re going to be calling many methods with the same interface, you can construct a `dbus.Interface` object and call methods on that, without needing to specify the interface again:

    import dbus
    bus = dbus.SystemBus()
    eth0 = bus.get_object('org.freedesktop.NetworkManager',
                          '/org/freedesktop/NetworkManager/Devices/eth0')
    eth0_dev_iface = dbus.Interface(eth0,
        dbus_interface='org.freedesktop.NetworkManager.Devices')
    props = eth0_dev_iface.getProperties()
    # props is the same as before

#### See also

See the example in `examples/example-client.py`. Before running it, you’ll need to run `examples/example-service.py` in the background or in another shell.

### Data types

Unlike Python, D-Bus is statically typed - each method has a certain signature representing the types of its arguments, and will not accept arguments of other types.

D-Bus has an introspection mechanism, which `dbus-python` tries to use to discover the correct argument types. If this succeeds, Python types are converted into the right D-Bus data types automatically, if possible; `TypeError` is raised if the type is inappropriate.

If the introspection mechanism fails (or the argument’s type is variant - see below), you have to provide arguments of the correct type. `dbus-python` provides Python types corresponding to the D-Bus data types, and a few native Python types are also converted to D-Bus data types automatically. If you use a type which isn’t among these, a `TypeError` will be raised telling you that `dbus-python` was unable to guess the D-Bus signature.

#### Basic types

The following basic data types are supported.

Python type | converted to D-Bus type | notes
--- | --- | ---
D-Bus proxy object | object path (signature ‘o’) | (+)
dbus.Interface
dbus.service.Object
`dbus.Boolean` | Boolean (signature ‘b’) | a subclass of `int`
`dbus.Byte` | byte (signature ‘y’)
`dbus.Int16` | 16-bit signed integer (‘n’)
`dbus.UInt16` | 16-bit unsigned integer (‘q’)
`dbus.Int32` | 32-bit signed integer (‘i’)
`dbus.UInt32` | 32-bit unsigned integer (‘u’) | a subclass of `long` (Python 2) a subclass of `int` (Python 3)
`dbus.Int64` | 64-bit signed integer (‘x’)
`dbus.UInt64` | 64-bit unsigned integer (‘t’)
`dbus.Double` | double-precision floating point (‘d’) | a subclass of `float`
`dbus.ObjectPath` | object path (‘o’) | a subclass of `str`
`dbus.Signature` | signature (‘g’)
`dbus.String` | string (‘s’) | a subclass of `unicode` (Python 2) a subclass of `str` (Python 3)
`dbus.UTF8String` | string (‘s’) | a subclass of `str`, only in Python 2
`bool` | Boolean (‘b’) |
`int` or subclass | 32-bit signed integer (‘i’) |
`long` or subclass | 64-bit signed integer (‘i’) | Python 2 only
`float` or subclass | double-precision floating point (‘d’) |
`bytes` or subclass | string (‘s’) | must be valid UTF-8
Python 2 `unicode` | string (‘s’) |
Python 3 `str`

(+): D-Bus proxy objects, exported D-Bus service objects and anything else with the special attribute `__dbus_object_path__`, which must be a string, are converted to their object-path. This might be useful if you’re writing an object-oriented API using dbus-python.

#### Basic type conversions

If introspection succeeded, `dbus-python` will also accept:

- for Boolean parameters, any object (converted as if via `int(bool(...))`)

- for byte parameters, a single-character string (converted as if via `ord()`)

- for byte and integer parameters, any integer (must be in the correct range)

- for object-path and signature parameters, any `str` or `unicode` subclass (the value must follow the appropriate syntax)

#### Container types

D-Bus supports four container types: array (a variable-length sequence of the same type), struct (a fixed-length sequence whose members may have different types), dictionary (a mapping from values of the same basic type to values of the same type), and variant (a container which may hold any D-Bus type, including another variant).

Arrays are represented by Python lists, or by `dbus.Array`, a subclass of `list`. When sending an array, if an introspected signature is available, that will be used; otherwise, if the `signature` keyword parameter was passed to the `Array` constructor, that will be used to determine the contents’ signature; otherwise, `dbus-python` will guess from the array’s first item.

The signature of an array is ‘ax’ where ‘x’ represents the signature of one item. For instance, you could also have ‘as’ (array of strings) or ‘a(ii)’ (array of structs each containing two 32-bit integers).

There’s also a type `dbus.ByteArray` which is a subclass of `bytes`, used as a more efficient representation of a D-Bus array of bytes (signature ‘ay’).

Structs are represented by Python tuples, or by `dbus.Struct`, a subclass of `tuple`. When sending a struct, if an introspected signature is available, that will be used; otherwise, if the `signature` keyword parameter was passed to the `Array` constructor, that will be used to determine the contents’ signature; otherwise, `dbus-python` will guess from the array’s first item.

The signature of a struct consists of the signatures of the contents, in parentheses - for instance ‘(is)’ is the signature of a struct containing a 32-bit integer and a string.

Dictionaries are represented by Python dictionaries, or by `dbus.Dictionary`, a subclass of `dict`. When sending a dictionary, if an introspected signature is available, that will be used; otherwise, if the `signature` keyword parameter was passed to the `Dictionary` constructor, that will be used to determine the contents’ key and value signatures; otherwise, `dbus-python` will guess from an arbitrary item of the `dict`.

The signature of a dictionary is ‘a{xy}’ where ‘x’ represents the signature of the keys (which may not be a container type) and ‘y’ represents the signature of the values. For instance, ‘a{s(ii)}’ is a dictionary where the keys are strings and the values are structs containing two 32-bit integers.

Variants are represented by setting the `variant_level` keyword argument in the constructor of any D-Bus data type to a value greater than 0 (`variant_level` 1 means a variant containing some other data type, `variant_level` 2 means a variant containing a variant containing some other data type, and so on). If a non-variant is passed as an argument but introspection indicates that a variant is expected, it’ll automatically be wrapped in a variant.

The signature of a variant is ‘v’.

#### Return values, and the `byte_arrays` and `utf8_strings` options

If a D-Bus method returns no value, the Python proxy method will return `None`.

If a D-Bus method returns one value, the Python proxy method will return that value as one of the `dbus.` types - by default, strings are returned as `dbus.String` (a subclass of Unicode) and byte arrays are returned as a `dbus.Array` of `dbus.Byte`.

If a D-Bus method returns multiple values, the Python proxy method will return a tuple containing those values.

If you want strings returned as `dbus.UTF8String` (a subclass of `bytes`) pass the keyword parameter `utf8_strings=True` to the proxy method. This mode is only available in Python 2.

If you want byte arrays returned as `dbus.ByteArray` (also a subclass of `bytes` - in practice, this is often what you want) pass the keyword parameter `byte_arrays=True` to the proxy method.

## Making asynchronous method calls

Asynchronous (non-blocking) method calls allow multiple method calls to be in progress simultaneously, and allow your application to do other work while it’s waiting for the results. To make asynchronous calls, you first need an event loop or “main loop”.

### Setting up an event loop

Currently, the only main loop supported by `dbus-python` is GLib.

`dbus-python` has a global default main loop, which is the easiest way to use this functionality. To arrange for the GLib main loop to be the default, use:

    from dbus.mainloop.glib import DBusGMainLoop

    DBusGMainLoop(set_as_default=True)

You must do this before connecting to the bus.

Actually starting the main loop is as usual for `pygi`:

    from gi.repository import GLib

    loop = GLib.MainLoop()
    loop.run()

While `loop.run()` is executing, GLib will run your callbacks when appropriate. To stop, call `loop.quit()`.

You can also set a main loop on a per-connection basis, by passing a main loop to the Bus constructor:

    import dbus
    from dbus.mainloop.glib import DBusGMainLoop

    dbus_loop = DBusGMainLoop()

    bus = dbus.SessionBus(mainloop=dbus_loop)

This isn’t very useful until we support more than one main loop, though.

#### Backwards compatibility: `dbus.glib`

In versions of `dbus-python` prior to 0.80, the way to set GLib as the default main loop was:

    import dbus.glib

Executing that import statement would automatically load the GLib main loop and make this the default. This is now deprecated, since it’s highly non-obvious, but may be useful if you want to write or understand backwards-compatible code.

#### The Qt main loop

PyQt v4.2 and later includes support for integrating dbus-python with the Qt event loop. To connect D-Bus to this main loop, call `dbus.mainloop.qt.DBusQtMainLoop` instead of `dbus.mainloop.glib.DBusGMainLoop`. Otherwise the Qt loop is used in exactly the same way as the GLib loop.

### Making asynchronous calls

To make a call asynchronous, pass two callables as keyword arguments `reply_handler` and `error_handler` to the proxy method. The proxy method will immediately return None. At some later time, when the event loop is running, one of these will happen: either

- the `reply_handler` will be called with the method’s return values as arguments; or

- the `error_handler` will be called with one argument, an instance of `DBusException` representing a remote exception.

#### See also

`examples/example-async-client.py` makes asynchronous method calls to the service provided by `examples/example-service.py` which return either a value or an exception. As for `examples/example-client.py`, you need to run `examples/example-service.py` in the background or in another shell first.

## Receiving signals

To receive signals, the Bus needs to be connected to an event loop - see section Setting up an event loop. Signals will only be received while the event loop is running.

### Signal matching

To respond to signals, you can use the `add_signal_receiver` method on Bus objects. This arranges for a callback to be called when a matching signal is received, and has the following arguments:

- a callable (the `handler_function`) which will be called by the event loop when the signal is received - its parameters will be the arguments of the signal

- the signal name, `signal_name`: here None (the default) matches all names

- the D-Bus interface, `dbus_interface`: again None is the default, and matches all interfaces

- a sender bus name (well-known or unique), `bus_name`: None is again the default, and matches all senders. Well-known names match signals from whatever application is currently the primary owner of that well-known name.

- a sender object path, `path`: once again None is the default and matches all object paths

`add_signal_receiver` also has keyword arguments `utf8_strings` and `byte_arrays` which influence the types used when calling the handler function, in the same way as the byte_arrays and utf8_strings options on proxy methods.

`add_signal_receiver` returns a `SignalMatch` object. Its only useful public API at the moment is a `remove` method with no arguments, which removes the signal match from the connection.

### Getting more information from a signal

You can also arrange for more information to be passed to the handler function. If you pass the keyword arguments `sender_keyword`, `destination_keyword`, `interface_keyword`, `member_keyword` or `path_keyword` to the `connect_to_signal` method, the appropriate part of the signal message will be passed to the handler function as a keyword argument: for instance if you use

    def handler(sender=None):
        print "got signal from %r" % sender

    iface.connect_to_signal("Hello", handler, sender_keyword='sender')

and a signal `Hello` with no arguments is received from `com.example.Foo`, the `handler` function will be called with `sender='com.example.Foo'`.

### String argument matching

If there are keyword parameters for the form `arg` n where n is a small non-negative number, their values must be Unicode strings (Python 2 `unicode` or Python 3 `str`) or UTF-8 bytestrings. The handler will only be called if that argument of the signal (numbered from zero) is a D-Bus string (in particular, not an object-path or a signature) with that value.

### Receiving signals from a proxy object

Proxy objects have a special method `connect_to_signal` which arranges for a callback to be called when a signal is received from the corresponding remote object. The parameters are:

- the name of the signal

- a callable (the handler function) which will be called by the event loop when the signal is received - its parameters will be the arguments of the signal

- the handler function, a callable: the same as for `add_signal_receiver`

- the keyword argument `dbus_interface` qualifies the name with its interface

dbus.Interface objects have a similar `connect_to_signal` method, but in this case you don’t need the `dbus_interface` keyword argument since the interface to use is already known.

The same extra keyword arguments as for `add_signal_receiver` are also available, and just like `add_signal_receiver`, it returns a SignalMatch.

You shouldn’t use proxy objects just to listen to signals, since they might activate the relevant service when created, but if you already have a proxy object in order to call methods, it’s often convenient to use it to add signal matches too.

### See also

`examples/signal-recipient.py` receives signals - it demonstrates general signal matching as well as `connect_to_signal`. Before running it, you’ll need to run `examples/signal-emitter.py` in the background or in another shell.

## Claiming a bus name

FIXME describe BusName - perhaps fix its API first?

### The unique-instance idiom

FIXME provide exemplary code, put it in examples

## Exporting objects

Objects made available to other applications over D-Bus are said to be exported. All subclasses of `dbus.service.Object` are automatically exported.

To export objects, the Bus needs to be connected to an event loop - see section Setting up an event loop. Exported methods will only be called, and queued signals will only be sent, while the event loop is running.

### Inheriting from `dbus.service.Object`

To export an object onto the Bus, just subclass `dbus.service.Object`. Object expects either a BusName or a Bus object, and an object-path, to be passed to its constructor: arrange for this information to be available. For example:

    class Example(dbus.service.Object):
        def __init__(self, object_path):
            dbus.service.Object.__init__(self, dbus.SessionBus(), path)

This object will automatically support introspection, but won’t do anything particularly interesting. To fix that, you’ll need to export some methods and signals too.

FIXME also mention dbus.gobject.ExportedGObject once I’ve written it

### Exporting methods with `dbus.service.method`

To export a method, use the decorator `dbus.service.method`. For example:

    class Example(dbus.service.Object):
        def __init__(self, object_path):
            dbus.service.Object.__init__(self, dbus.SessionBus(), path)

        @dbus.service.method(dbus_interface='com.example.Sample',
                             in_signature='v', out_signature='s')
        def StringifyVariant(self, variant):
            return str(variant)

The `in_signature` and `out_signature` are D-Bus signature strings as described in Data Types.

As well as the keywords shown, you can pass `utf8_strings` and `byte_arrays` keyword arguments, which influence the types which will be passed to the decorated method when it’s called via D-Bus, in the same way that the byte_arrays and utf8_strings options affect the return value of a proxy method.

You can find a simple example in `examples/example-service.py`, which we used earlier to demonstrate `examples/example-client.py`.

#### Finding out the caller’s bus name

The `method` decorator accepts a `sender_keyword` keyword argument. If you set that to a string, the unique bus name of the sender will be passed to the decorated method as a keyword argument of that name:

    class Example(dbus.service.Object):
        def __init__(self, object_path):
            dbus.service.Object.__init__(self, dbus.SessionBus(), path)

        @dbus.service.method(dbus_interface='com.example.Sample',
                             in_signature='', out_signature='s',
                             sender_keyword='sender')
        def SayHello(self, sender=None):
            return 'Hello, %s!' % sender
            # -> something like 'Hello, :1.1!'

#### Asynchronous method implementations

FIXME and also add an example, perhaps examples/example-async-service.py

### Emitting signals with `dbus.service.signal`

To export a signal, use the decorator `dbus.service.signal`; to emit that signal, call the decorated method. The decorated method can also contain code which will be run when called, as usual. For example:

    class Example(dbus.service.Object):
        def __init__(self, object_path):
            dbus.service.Object.__init__(self, dbus.SessionBus(), path)

        @dbus.service.signal(dbus_interface='com.example.Sample',
                             signature='us')
        def NumberOfBottlesChanged(self, number, contents):
            print "%d bottles of %s on the wall" % (number, contents)

    e = Example('/bottle-counter')
    e.NumberOfBottlesChanged(100, 'beer')
    # -> emits com.example.Sample.NumberOfBottlesChanged(100, 'beer')
    #    and prints "100 bottles of beer on the wall"

The signal will be queued for sending when the decorated method returns - you can prevent the signal from being sent by raising an exception from the decorated method (for instance, if the parameters are inappropriate). The signal will only actually be sent when the event loop next runs.

#### Example

`examples/example-signal-emitter.py` emits some signals on demand when one of its methods is called. (In reality, you’d emit a signal when some sort of internal state changed, which may or may not be triggered by a D-Bus method call.)

## License for this document

Copyright 2006-2007 Collabora Ltd.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.