xdist: pytest distributed testing plugin

The pytest-xdist plugin extends pytest with some unique test execution modes:

  • Looponfail: run your tests repeatedly in a subprocess. After each run, pytest waits until a file in your project changes and then re-runs the previously failing tests. This is repeated until all tests pass. At this point a full run is again performed.
  • multiprocess Load-balancing: if you have multiple CPUs or hosts you can use them for a combined test run. This allows to speed up development or to use special resources of remote machines.
  • Multi-Platform coverage: you can specify different Python interpreters or different platforms and run tests in parallel on all of them.

Before running tests remotely, pytest efficiently “rsyncs” your program source code to the remote place. All test results are reported back and displayed to your local terminal. You may specify different Python versions and interpreters.

Installation of xdist plugin

Install the plugin with:

easy_install pytest-xdist

# or

pip install pytest-xdist

or use the package in develop/in-place mode with a checkout of the pytest-xdist repository

python setup.py develop

Usage examples

Speed up test runs by sending tests to multiple CPUs

To send tests to multiple CPUs, type:

py.test -n NUM

Especially for longer running tests or tests requiring a lot of I/O this can lead to considerable speed ups.

Running tests in a Python subprocess

To instantiate a Python-2.4 subprocess and send tests to it, you may type:

py.test -d --tx popen//python=python2.4

This will start a subprocess which is run with the “python2.4” Python interpreter, found in your system binary lookup path.

If you prefix the –tx option value like this:

py.test -d --tx 3*popen//python=python2.4

then three subprocesses would be created and the tests will be distributed to three subprocesses and run simultanously.

Running tests in looponfailing mode

For refactoring a project with a medium or large test suite you can use the looponfailing mode. Simply add the --f option:

py.test -f

and pytest will run your tests. Assuming you have failures it will then wait for file changes and re-run the failing test set. File changes are detected by looking at looponfailingroots root directories and all of their contents (recursively). If the default for this value does not work for you you can change it in your project by setting a configuration option:

# content of a pytest.ini, setup.cfg or tox.ini file
looponfailroots = mypkg testdir

This would lead to only looking for file changes in the respective directories, specified relatively to the ini-file’s directory.

Sending tests to remote SSH accounts

Suppose you have a package mypkg which contains some tests that you can successfully run locally. And you also have a ssh-reachable machine myhost. Then you can ad-hoc distribute your tests by typing:

py.test -d --tx ssh=myhostpopen --rsyncdir mypkg mypkg

This will synchronize your mypkg package directory with a remote ssh account and then collect and run your tests at the remote side.

You can specify multiple --rsyncdir directories to be sent to the remote side.

Sending tests to remote Socket Servers

Download the single-module socketserver.py Python program and run it like this:

python socketserver.py

It will tell you that it starts listening on the default port. You can now on your home machine specify this new socket host with something like this:

py.test -d --tx socket= --rsyncdir mypkg mypkg

Running tests on many platforms at once

The basic command to run tests on multiple platforms is:

py.test --dist=each --tx=spec1 --tx=spec2

If you specify a windows host, an OSX host and a Linux environment this command will send each tests to all platforms - and report back failures from all platforms at once. The specifications strings use the xspec syntax.

Specifying test exec environments in an ini file

pytest (since version 2.0) supports ini-style configuration. For example, you could make running with three subprocesses your default:

addopts = -n3

You can also add default environments like this:

addopts = --tx ssh=myhost//python=python2.5 --tx ssh=myhost//python=python2.6

and then just type:

py.test --dist=each

to run tests in each of the environments.

Specifying “rsync” dirs in an ini-file

In a tox.ini or setup.cfg file in your root project directory you may specify directories to include or to exclude in synchronisation:

rsyncdirs = . mypkg helperpkg
rsyncignore = .hg

These directory specifications are relative to the directory where the configuration file was found.