Writing Basic Tests with Python

Starting point: we’ll assume you have a basic ament_python package set up already and you want to add some tests to it.

If you are using ament_cmake_python, refer to the the ament_cmake_python docs for how to make tests dicoverable. The test contents and invocation with colcon remain the same.

Package Setup


Your setup.py must have a test dependency on pytest within the call to setup(...):


Test Files and Folders

Your test code needs to go in a folder named tests in the root of your package.

Any file that contains tests that you want to run must have the pattern test_FOO.py where FOO can be replaced with anything.

Example package layout:


Test Contents

You can now write tests to your heart’s content. There are plenty of resources on pytest, but in short, you can write functions with the test_ prefix and include whatever assert statements you’d like.

def test_math():
    assert 2 + 2 == 5   # This should fail for most mathematical systems

Running Tests

See the tutorial on how to run tests from the command line for more information on running the tests and inspecting the test results.

Special Commands

Beyond the standard colcon testing commands you can also specify arguments to the pytest framework from the command line with the --pytest-args flag. For example, you can specify the name of the function to run with

colcon test --packages-select <name-of-pkg> --pytest-args -k name_of_the_test_function

To see the pytest output while running the tests, use these flags:

colcon test --event-handlers console_cohesion+