Running Tests in Parallel with Multiple Processes


New in version 0.3

Use the mp plugin to enable distribution of tests across multiple processes. Doing this may speed up your test run if your tests are heavily IO or CPU bound. But it imposes an overhead cost that is not trivial, and it complicates the use of test fixtures and may conflict with plugins that are not designed to work with it.


To activate the plugin, include the plugin module in the plugins list in [unittest] section in a config file:

plugins =

Or pass the module with the --plugin command-line option:


Then configure the number of processes to run. You can do that either with the -N option:

nose2 -N 2

or by setting processes in the [multiprocess] section of a config file:

processes = 2


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:

bind_address =

This will bind to port 1024 of Also:

bind_address =

will bind to any random open port on Any internet address or host-name which python can recognize as such, bind, and connect is acceptable. While 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 setUpClassmust 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)

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):

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)

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)

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.


Event fired to notify plugins that multiprocess testing will occur


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):
class, result, runner, plugins, connection, **metadata)[source]

Event fired at start and end of subprocess execution.


Test loader instance


Test result


List of plugins loaded in the subprocess.


The multiprocessing.Connection instance that the subprocess uses for communication with the main process.


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.

Loading Tests

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), may attempt to built a 2gig string, and possibly throw a MemoryError exception. The related bug is


Enable this Plugin

This plugin is built-in, but not loaded by default.

Even if you specify always-on = True in the configuration, it will not run unless you also enable it. You can do so by putting the following in a unittest.cfg or nose2.cfg file

plugins =

The plugins parameter may contain a list of plugin names, including

Configuration [multiprocess]

















Sample configuration

The default configuration is equivalent to including the following in a unittest.cfg file.

always-on = False
processes = 0
test-run-timeout = 60.0

Command-line options

-N DEFAULT, --processes DEFAULT

Number of processes used to run tests (0 = auto)

Plugin class reference: MultiProcess

class*args, **kwargs)[source]
property procs

Get the appropriate number of procs for self.procs if self._procs is 0.