python-dbusmock
Purpose
With this program/Python library you can easily create mock objects on D-Bus. This is useful for writing tests for software which talks to D-Bus services such as upower, systemd, logind, gnome-session or others, and it is hard (or impossible without root privileges) to set the state of the real services to what you expect in your tests.
Suppose you want to write tests for gnome-settings-daemon’s power
plugin, or another program that talks to upower. You want to verify that
after the configured idle time the program suspends the machine. So your
program calls org.freedesktop.UPower.Suspend()
on the system D-Bus.
Now, your test suite should not really talk to the actual system D-Bus
and the real upower; a make check
that suspends your machine will not
be considered very friendly by most people, and if you want to run this
in continuous integration test servers or package build environments,
chances are that your process does not have the privilege to suspend, or
there is no system bus or upower to begin with. Likewise, there is no
way for an user process to forcefully set the system/seat idle flag in
logind, so your tests cannot set up the expected test environment on the
real daemon.
That’s where mock objects come into play: They look like the real API (or at least the parts that you actually need), but they do not actually do anything (or only some action that you specify yourself). You can configure their state, behaviour and responses as you like in your test, without making any assumptions about the real system status.
When using a local system/session bus, you can do unit or integration
testing without needing root privileges or disturbing a running system.
The Python API offers some convenience functions like
start_session_bus()
and start_system_bus()
for this, in a
DBusTestCase
class (subclass of the standard unittest.TestCase
) or
alternatively as a @pytest.fixture
.
You can use this with any programming language, as you can run the
mocker as a normal program. The actual setup of the mock (adding
objects, methods, properties, and signals) all happen via D-Bus methods
on the org.freedesktop.DBus.Mock
interface. You just don’t have the
convenience D-Bus launch API that way.
Simple example using Python’s unittest
Picking up the above example about mocking upower’s Suspend()
method,
this is how you would set up a mock upower in your test case:
import subprocess
import dbus
import dbusmock
class TestMyProgram(dbusmock.DBusTestCase):
@classmethod
def setUpClass(cls):
cls.start_system_bus()
cls.dbus_con = cls.get_dbus(system_bus=True)
def setUp(self):
self.p_mock = self.spawn_server('org.freedesktop.UPower',
'/org/freedesktop/UPower',
'org.freedesktop.UPower',
system_bus=True,
stdout=subprocess.PIPE)
# Get a proxy for the UPower object's Mock interface
self.dbus_upower_mock = dbus.Interface(self.dbus_con.get_object(
'org.freedesktop.UPower', '/org/freedesktop/UPower'),
dbusmock.MOCK_IFACE)
self.dbus_upower_mock.AddMethod('', 'Suspend', '', '', '')
def tearDown(self):
self.p_mock.stdout.close()
self.p_mock.terminate()
self.p_mock.wait()
def test_suspend_on_idle(self):
# run your program in a way that should trigger one suspend call
# now check the log that we got one Suspend() call
self.assertRegex(self.p_mock.stdout.readline(), b'^[0-9.]+ Suspend$')
Let’s walk through:
We derive our tests from
dbusmock.DBusTestCase
instead ofunittest.TestCase
directly, to make use of the convenience API to start a local system bus.setUpClass()
starts a local system bus, and makes a connection to it available to all methods asdbus_con
.True
means that we connect to the system bus, not the session bus. We can use the same bus for all tests, so doing this once insetUpClass()
instead ofsetUp()
is enough.setUp()
spawns the mock D-Bus server process for an initial/org/freedesktop/UPower
object with anorg.freedesktop.UPower
D-Bus interface on the system bus. We capture its stdout to be able to verify that methods were called.We then call
org.freedesktop.DBus.Mock.AddMethod()
to add aSuspend()
method to our new object to the default D-Bus interface. This will not do anything (except log its call to stdout). It takes no input arguments, returns nothing, and does not run any custom code.tearDown()
stops our mock D-Bus server again. We do this so that each test case has a fresh and clean upower instance, but of course you can also set up everything insetUpClass()
if tests do not interfere with each other on setting up the mock.test_suspend_on_idle()
is the actual test case. It needs to run your program in a way that should trigger one suspend call. Your program will try to callSuspend()
, but as that’s now being served by our mock instead of upower, there will not be any actual machine suspend. Our mock process will log the method call together with a time stamp; you can use the latter for doing timing related tests, but we just ignore it here.
Simple example using pytest
The same functionality as above but instead using the pytest fixture provided by this package.
import subprocess
import dbus
import pytest
import dbusmock
@pytest.fixture
def upower_mock(dbusmock_system):
p_mock = dbusmock_system.spawn_server(
'org.freedesktop.UPower',
'/org/freedesktop/UPower',
'org.freedesktop.UPower',
system_bus=True,
stdout=subprocess.PIPE)
# Get a proxy for the UPower object's Mock interface
dbus_upower_mock = dbus.Interface(dbusmock_system.get_dbus(True).get_object(
'org.freedesktop.UPower',
'/org/freedesktop/UPower'
), dbusmock.MOCK_IFACE)
dbus_upower_mock.AddMethod('', 'Suspend', '', '', '')
yield p_mock
p_mock.stdout.close()
p_mock.terminate()
p_mock.wait()
def test_suspend_on_idle(upower_mock):
# run your program in a way that should trigger one suspend call
# now check the log that we got one Suspend() call
assert upower_mock.stdout.readline() == b'^[0-9.]+ Suspend$'
Let’s walk through:
We import the
dbusmock_system
fixture from dbusmock which provides us with a system bus started for our test case wherever thedbusmock_system
argument is used by a test case and/or a pytest fixture.The
upower_mock
fixture spawns the mock D-Bus server process for an initial/org/freedesktop/UPower
object with anorg.freedesktop.UPower
D-Bus interface on the system bus. We capture its stdout to be able to verify that methods were called.We then call
org.freedesktop.DBus.Mock.AddMethod()
to add aSuspend()
method to our new object to the default D-Bus interface. This will not do anything (except log its call to stdout). It takes no input arguments, returns nothing, and does not run any custom code.This mock server process is yielded to the test function that uses the
upower_mock
fixture - once the test is complete the process is terminated again.test_suspend_on_idle()
is the actual test case. It needs to run your program in a way that should trigger one suspend call. Your program will try to callSuspend()
, but as that’s now being served by our mock instead of upower, there will not be any actual machine suspend. Our mock process will log the method call together with a time stamp; you can use the latter for doing timing related tests, but we just ignore it here.
Simple example from shell
We use the actual session bus for this example. You can use
dbus-run-session
to start a private one as well if you want, but that
is not part of the actual mocking.
So let’s start a mock at the D-Bus name com.example.Foo
with an
initial “main” object on path /, with the main D-Bus interface
com.example.Foo.Manager
:
python3 -m dbusmock com.example.Foo / com.example.Foo.Manager
On another terminal, let’s first see what it does:
gdbus introspect --session -d com.example.Foo -o /
You’ll see that it supports the standard D-Bus Introspectable
and
Properties
interfaces, as well as the org.freedesktop.DBus.Mock
interface for controlling the mock, but no “real” functionality yet.
So let’s add a method:
gdbus call --session -d com.example.Foo -o / -m org.freedesktop.DBus.Mock.AddMethod '' Ping '' '' ''
Now you can see the new method in introspect
, and call it:
gdbus call --session -d com.example.Foo -o / -m com.example.Foo.Manager.Ping
The mock process in the other terminal will log the method call with a
time stamp, and you’ll see something like 1348832614.970 Ping
.
Now add another method with two int arguments and a return value and call it:
gdbus call --session -d com.example.Foo -o / -m org.freedesktop.DBus.Mock.AddMethod \
'' Add 'ii' 'i' 'ret = args[0] + args[1]'
gdbus call --session -d com.example.Foo -o / -m com.example.Foo.Manager.Add 2 3
This will print (5,)
as expected (remember that the return value is
always a tuple), and again the mock process will log the Add method
call.
You can do the same operations in e. g. d-feet or any other D-Bus language binding.
Interactive debugging
It’s possible to use dbus-mock to run interactive sessions using something like:
python3 -m dbusmock com.example.Foo / com.example.Foo.Manager -e $SHELL
Where a shell session with the defined mocks is set and others can be added.
Or more complex ones such as:
python3 -m dbusmock --session -t upower -e \
python3 -m dbusmock com.example.Foo / com.example.Foo.Manager -e \
gdbus introspect --session -d com.example.Foo -o /
Logging
Usually you want to verify which methods have been called on the mock with which arguments. There are three ways to do that:
By default, the mock process writes the call log to stdout.
You can call the mock process with the
-l
/--logfile
argument, or specify a log file object in thespawn_server()
method if you are using Python.You can use the
GetCalls()
,GetMethodCalls()
andClearCalls()
methods on theorg.freedesktop.DBus.Mock
D-Bus interface to get an array of tuples describing the calls.
Templates
Some D-Bus services are commonly used in test suites, such as UPower or
NetworkManager. python-dbusmock provides “templates” which set up the
common structure of these services (their main objects, properties, and
methods) so that you do not need to carry around this common code, and
only need to set up the particular properties and specific D-Bus objects
that you need. These templates can be parameterized for common
customizations, and they can provide additional convenience methods on
the org.freedesktop.DBus.Mock
interface to provide more abstract
functionality like “add a battery”.
For example, for starting a server with the upower
template in
Python you can run
(self.p_mock, self.obj_upower) = self.spawn_server_template(
'upower', {'OnBattery': True}, stdout=subprocess.PIPE)
or load a template into an already running server with the
AddTemplate()
method; this is particularly useful if you are not using
Python:
python3 -m dbusmock --system org.freedesktop.UPower /org/freedesktop/UPower org.freedesktop.UPower
gdbus call --system -d org.freedesktop.UPower -o /org/freedesktop/UPower -m org.freedesktop.DBus.Mock.AddTemplate 'upower' '{"OnBattery": <true>}'
This creates all expected properties such as DaemonVersion
, and
changes the default for one of them (OnBattery
) through the (optional)
parameters dict.
If you do not need to specify parameters, you can do this in a simpler way with
python3 -m dbusmock --template upower
The template does not create any devices by default. You can add some with the template’s convenience methods like
ac_path = self.dbusmock.AddAC('mock_AC', 'Mock AC')
bt_path = self.dbusmock.AddChargingBattery('mock_BAT', 'Mock Battery', 30.0, 1200)
or calling AddObject()
yourself with the desired properties, of
course.
Templates commonly implement some non-trivial functionality with actual Python
methods and the standard dbus-python
@dbus.service.method
decorator.
To build your own template, you can copy
dbusmock/templates/SKELETON to your
new template file name and replace CHANGEME
with the actual code/values.
Look at dbusmock/templates/upower.py for
a real-life implementation.
A template can be loaded from these locations:
Provide a path to its
.py
file. This is intended for running tests out of git/build trees with very project specific or unstable templates.From
$XDG_DATA_DIRS/python-dbusmock/templates/
name.py
. This is intended for shipping reusable templates in distribution development packages. Load them by module name.python-dbusmock ships a set of widely applicable templates which are collaboratively maintained, like the
upower
one in the example above. Load them by module name.
More Examples
Have a look at the test suite for two real-live use cases:
tests/test_upower.py
simulates upowerd, in a more complete way than in above example and using theupower
template. It verifies thatupower --dump
is convinced that it’s talking to upower.tests/test_api.py
runs a mock on the session bus and exercises all available functionality, such as adding additional objects, properties, multiple methods, input arguments, return values, code in methods, sending signals, raising exceptions, and introspection.
Documentation
The dbusmock
module has extensive documentation built in, which you
can read with e. g. pydoc3 dbusmock
or online at
https://martinpitt.github.io/python-dbusmock/
pydoc3 dbusmock.DBusMockObject
shows the D-Bus API of the mock object,
i. e. methods like AddObject()
, AddMethod()
etc. which are used to
set up your mock object.
pydoc3 dbusmock.DBusTestCase
shows the convenience Python API for
writing test cases with local private session/system buses and launching
the server.
pydoc3 dbusmock.templates
shows all available templates.
pydoc3 dbusmock.templates.NAME
shows the documentation and available
parameters for the NAME
template.
python3 -m dbusmock --help
shows the arguments and options for running
the mock server as a program.
Development
python-dbusmock is hosted on https://github.com/martinpitt/python-dbusmock
Run the unit tests with python3 -m unittest
or pytest
.
In CI, the unit tests run in containers. You can run them locally with e.g.
tests/run registry.fedoraproject.org/fedora:latest
Check the unit-tests GitHub workflow for the operating systems/container images on which python-dbusmock is tested and supported.
To debug failures interactively, run
DEBUG=1 tests/run [image]
which will sleep on failures. You can then attach to the running container
image with e.g. podman exec -itl bash
. The /source
directory is mounted from the
host, i.e. edit files in your normal git checkout outside of the container, and
re-run all tests in the container shell like above. You can also run a specific
test:
python3 -m unittest tests.test_api.TestAPI.test_onearg_ret