Running the Hypothesis Python Tests

This is part of a series of posts on Python unit testing.

This first post covers running the tests and if you want to try out the example commands you should have a Hypothesis dev install.

The easiest way to run all of the tests in the hypothesis/h repo (including both Python and JavaScript tests) is by running the make test command:

$ make test

This will run all of the Python and JavaScript tests at once. This is the command that is run on Travis CI to verify each new commit that’s pushed to GitHub. But when working on the Python code I usually run the tests using one of these two commands instead:

# To run the h tests:
$ tox -e py27-h tests/h
... (test output)

# To run the memex tests:
$ tox -e py27-memex tests/memex
... (test output)

The reason to use these commands instead of make test is that you can pass lots of useful options to them (see below) that you can’t pass to make test, for example to only run certain tests rather than all of them. I might run make test once as a final check before submitting a pull request, or after setting up a new development environment, but the rest of the time I use the tox commands above.

Before we have a look at all the useful options you can pass to tox, lets first dig in to how we get from make test to tox -e py27-h tests/h.


The make test command is provided by h’s Makefile (see GNU Make). If you look inside h’s Makefile you can see that the command that make test runs to run the Python part of the tests is tox


You can run the tox command that make test runs manually, this will run all all the Python tests but not the JavaScript ones:

$ tox
(Runs all the Python tests)

tox is a tool that creates and manages clean room Python virtual environments for running the tests in. When you run tox it creates virtual environments in the .tox directory, installs h and memex’s dependencies in the virtualenvs, and then runs the tests in them.

Sometimes a development environment has older or newer versions of h’s dependencies installed than what is actually specified in h’s requirements, or has additional packages installed. Tox’s use of clean virtual environments to run the tests in avoids differences between different developers’ environments, or between dev envs and production, from producing different test results.

You can see the virtualenvs that tox has created by running tox --listenvs or by looking in the .tox directory:

$ ls .tox
dist/  log/  py27-h/  py27-memex/

dist and log are used by tox internally, py27-h and py27-memex are the two test environments that h uses. Tox runs the tests in the tests/h directory in the py27-h environment and runs the tests in tests/memex in the py27-memex environment.

(There’s also an environment called functional for the functional tests but make test and tox don’t run the functional tests by default. To run the functional tests locally do tox -e functional tests/functional.)

The full configuration for tox is in the tox.ini file. This is where we define the different test virtualenvs, which dependencies to install into each, and which tests to run in each, among other things. In this file you can see that the command that tox actually runs in order to run the tests is pytest. Pytest is the test framework that we use to write h’s Python tests.

Tip: To save time tox doesn’t normally recreate these virtualenvs each time you run the tests. It creates the virtualenvs once, installs the dependencies, and then reuses them each time. If h’s specified dependencies change then tox is supposed to update the virtualenv but sometimes it doesn’t and you’ll have tests failing because of a missing or out of date dependency. If this happens you can force tox to recreate the virtualenvs by running tox --recreate. (Alternatively just delete the .tox directory then run tox.)


pytest is the command that actually runs the Python tests. Pytest is the framework that we use to write Python tests in Hypothesis. If you’re looking for documentation on how to write Hypothesis tests, the pytest docs are the main place to look.

Since we’re running pytest via tox rather than directly we have to pass arguments to pytest through tox. Anything after a standalone -- in the tox command will be passed to pytest. For example tox --help is going to print tox’s help documentation, not pytest’s, to print pytest’s help you would do:

$ tox -- --help

Any arguments that you give to tox or pytest on the command line will override the default arguments given in the tox.ini file. By default the tox.ini file runs two commands, tox -e py27-h tests/h and then tox -e py27-memex tests/memex. The -e py27-h tells tox to use the py27-h test virtualenv, and the tests/h part tells tox to tell pytest to run only the tests in the tests/h directory. You can’t run the tests/h tests in the py27-memex virtualenv because that virtualenv has the dependencies for the tests/memex tests installed in it, not the dependencies for the tests/h tests, so tox -e py27-memex tests/h will crash. So when passing custom options on the command line you always need to pass the virtualenv name and the path to the tests as well, it’s always either tox -e py27-h tests/h or tox -e py27-memex tests/memex. (Tip: create aliases for these commands in your shell configuration.)

Tip: Some people like to run pytest directly because it starts up quicker than tox and because it’s easier to pass arguments to pytest this way. The easiest way to do this is to run one of the copies of pytest that tox has installed in one of its virtualenvs, for example: .tox/py27-h/bin/pytest tests/h/. (If you use this command often then it’s probably worth creating a shell alias for it too.)

Useful command line options for pytest

To run just the h tests (and not the memex ones):

$ tox -e py27-h tests/h

To run just a single test package just change the path:

$ tox -e py27-h tests/h/views

Run just a single test module give the path to the module file including the .py ending:

$ tox -e py27-h tests/h/views/

Run just a single test class add :: followed by the name of the test class:

$ tox -e py27-h tests/h/views/

To run just a single test method add another :: before the name of the method:

$ tox -e py27-h tests/h/views/

Tip: Running the tests with the --verbose argument to pytest will print out all the test method names in a format suitable for copy-pasting into the above command to run a single test individually:

$ tox -e py27-h tests/h -- --verbose
tests/h/ PASSED
tests/h/ PASSED
tests/h/ PASSED

The -k argument to pytest lets you specify a string pattern and pytest will run only the tests whose name matches that pattern. This can be more convenient than typing in the full path. For example to run just one test class:

$ tox -e py27-h tests/h -- -k TestHasFeatureFlagPredicate

You can also use logic like not, and and or in with -k, for details see the -k docs.

The --failed-first / --ff argument runs the tests that failed the last time first, and then runs the rest of the tests. It works well in combination with --exitfirst / -x which causes pytest to stop after the first test fail. As long as you have a failing test it’ll run just that test and then stop, as soon as the failing test passes it’ll continue running the rest of the tests:

$ tox -e py27-h tests/h -- --failed-first --exitfirst

Some other useful arguments to pass after the --:

To specify a default set of options to always pass to pytest you can set the PYTEST_ADDOPTS environment variable, for example:

$ export PYTEST_ADDOPTS="--verbose --exitfirst --failed-first --showlocals"

(This works well in combination with direnv.)

That’s everything about how to run the Python tests (whew!). The next post will cover how to debug a failing test…