Usage
To activate the plugin, include the plugin module in the plugins list
in [unittest] section in a config file:
[unittest]
plugins = nose2.plugins.mp
Or pass the module with the --plugin command-line option:
nose2 --plugin=nose2.plugins.mp
Then configure the number of processes to run. You can do that either
with the -N option:
or by setting processes in the [multiprocess] section of a
config file:
[multiprocess]
processes = 2
Note
If you make the plugin always active by setting always-on in
the [multiprocess] section of a config file, but do not set
processes or pass -N, the number of processes
defaults to the number of CPUs available. Also note that a value of 0 will
set the actual number of processes to the number of CPUs on the computer.
Should one wish to specify the use of internet sockets for
interprocess communications, specify the bind_address
setting in the [multiprocess] section of the config file,
for example:
[multiprocess]
bind_address = 127.0.0.1:1024
This will bind to port 1024 of 127.0.0.1. Also:
[multiprocess]
bind_address = 127.1.2.3
will bind to any random open port on 127.1.2.3. Any internet
address or host-name which python can recognize as such, bind, and
connect is acceptable. While 0.0.0.0 can be use for listening,
it is not necessarily an address to which the OS can connect. When
the port address is 0 or omitted, a random open port is used. If
the setting is omitted or blank, then sockets are not used unless
nose is being executed on Windows. In which case, an address on
the loop back interface and a random port are used. Whenever used,
processes employ a random shared key for authentication.
Guidelines for Test Authors
Not every test suite will work well, or work at all, when run in
parallel. For some test suites, parallel execution makes no sense. For
others, it will expose bugs and ordering dependencies in test cases and
test modules.
Overhead Cost
Starting subprocesses and dispatching tests takes time. A test run
that includes a relatively small number of tests that are not I/O or
CPU bound (or calling time.sleep()) is likely to be slower when run
in parallel.
As of this writing, for instance, nose2’s test suite
takes about 10 times as long to run when using multiprocessing, due to
the overhead cost.
Shared Fixtures
The individual test processes do not share state or data after
launch. This means tests that share a fixture – tests that are loaded
from modules where setUpModule is defined, and tests in test
classes that define setUpClass – must all be dispatched to the
same process at the same time. So if you use these kinds of fixtures,
your test runs may be less parallel than you expect.
Tests Load Twice
Test cases may not be pickleable, so nose2 can’t transmit them
directly to its test runner processes. Tests are distributed by
name. This means that tests always load twice – once in the main
process, during initial collection, and then again in the test runner
process, where they are loaded by name. This may be problematic for
some test suites.
Random Execution Order
Tests do not execute in the same order when run in parallel. Results
will be returned in effectively random order, and tests in the same
module (as long as they do not share fixtures) may execute in any
order and in different processes. Some test suites have ordering
dependencies, intentional or not, and those that do will fail randomly
when run with this plugin.
Guidelines for Plugin Authors
The MultiProcess plugin is designed to work with other plugins, but
other plugins may have to return the favor, especially if they load
tests or care about something that happens during test execution.
New Methods
The MultiProcess plugin adds a few plugin hooks that other plugins can
use to set themselves up for multiprocess test runs. Plugins don’t
have to do anything special to register for these hooks; just
implement the methods as normal.
-
registerInSubprocess(self, event)
- Parameters:
event – nose2.plugins.mp.RegisterInSubprocessEvent
The registerInSubprocess hook is called after plugin
registration to enable plugins that need to run in subprocesses to
register that fact. The most common thing to do, for plugins that
need to run in subprocesses, is:
def registerInSubprocess(self, event):
event.pluginClasses.append(self.__class__)
It is not required that plugins append their own class. If for some
reason there is a different plugin class, or set of classes, that
should run in the test-running subprocesses, add that class or
those classes instead.
-
startSubprocess(self, event)
- Parameters:
event – nose2.plugins.mp.SubprocessEvent
The startSubprocess hook fires in each test-running subprocess
after it has loaded its plugins but before any tests are executed.
Plugins can customize test execution here in the same way as in
startTestRun(), by setting event.executeTests, and
prevent test execution by setting event.handled to True and
returning False.
-
stopSubprocess(self, event)
- Parameters:
event – nose2.plugins.mp.SubprocessEvent
The stopSubprocess event fires just before each test running
subprocess shuts down. Plugins can use this hook for any
per-process finalization that they may need to do.
The same event instance is passed to startSubprocess and
stopSubprocess, which enables plugins to use that event’s
metadata to communicate state or other information from the
start to the stop hooks, if needed.
New Events
The MultiProcess plugin’s new hooks come with custom event classes.
-
class nose2.plugins.mp.RegisterInSubprocessEvent(**metadata)[source]
Event fired to notify plugins that multiprocess testing will occur
-
pluginClasses
Add a plugin class to this list to cause the plugin to be
instantiated in each test-running subprocess. The most common
thing to do, for plugins that need to run in subprocesses, is:
def registerInSubprocess(self, event):
event.pluginClasses.append(self.__class__)
-
class nose2.plugins.mp.SubprocessEvent(loader, result, runner, plugins, connection, **metadata)[source]
Event fired at start and end of subprocess execution.
-
loader
Test loader instance
-
result
Test result
-
plugins
List of plugins loaded in the subprocess.
-
connection
The multiprocessing.Connection instance that the
subprocess uses for communication with the main process.
-
executeTests
Callable that will be used to execute tests. Plugins may set
this attribute to wrap or otherwise change test execution. The
callable must match the signature:
def execute(suite, result):
...
Stern Warning
All event attributes, including ``event.metadata``, must be
pickleable. If your plugin sets any event attributes or puts anything
into event.metadata, it is your responsibility to ensure that
anything you can possibly put in is pickleable.
Do I Really Care?
If you answer yes to any of the following questions, then your
plugin will not work with multiprocess testing without modification:
Does your plugin load tests?
Does your plugin capture something that happens during test execution?
Does your plugin require user interaction during test execution?
Does your plugin set executeTests in startTestRun?
Here’s how to handle each of those cases.
Capturing Test Execution State
Implement registerInSubprocess() as suggested to enable your
plugin in the test runner processes.
Be wary of setting event.metadata unconditionally. Your plugin
will execute in the main process and in the test runner processes,
and will see setTestOutcome() and testOutcome() events
in both processes. If you unconditionally set a key in
event.metadata, the plugin instance in the main process will
overwrite anything set in that key by the instance in the
subprocess.
If you need to write something to a file, implement
stopSubprocess() to write a file in each test runner process.
Overriding Test Execution
Implement registerInSubprocess() as suggested to enable your
plugin in the test runner processes and make a note that your plugin
is running under a multiprocess session.
When running multiprocess, do not set event.executeTests in
startTestRun() – instead, set it in startSubprocess()
instead. This will allow the multiprocess plugin to install its test
executor in the main process, while your plugin takes over test
execution in the test runner subprocesses.
Interacting with Users
You are probably safe because as a responsible plugin author you are
already firing the interaction hooks (beforeInteraction(),
afterInteraction()) around your interactive bits, and skipping
them when the beforeInteraction() hook returns False and sets
event.handled.
If you’re not doing that, start!
Possible Issues On Windows
On windows, there are a few known bugs with respect to multiprocessing.
Python on windows does not use fork(). It bootstraps from a
separate interpreter invocation. In certain contexts, the “value” for
a parameter will be taken as a “count” and subprocess use this to build
the flag for the command-line. E.g., If this value is 2 billion
(like a hash seed), subprocess.py may attempt to built a 2gig string,
and possibly throw a MemoryError exception. The related bug is
http://bugs.python.org/issue20954.