Contributing to coverage.py

I welcome contributions to coverage.py. Over the years, dozens of people have provided patches of various sizes to add features or fix bugs. This page should have all the information you need to make a contribution.

One source of history or ideas are the bug reports against coverage.py. There you can find ideas for requested features, or the remains of rejected ideas.

Before you begin

If you have an idea for coverage.py, run it by me before you begin writing code. This way, I can get you going in the right direction, or point you to previous work in the area. Things are not always as straightforward as they seem, and having the benefit of lessons learned by those before you can save you frustration.

Getting the code

The coverage.py code is hosted on a GitHub repository at https://github.com/nedbat/coveragepy. To get a working environment, follow these steps:

  1. Create a Python 3.7 virtualenv to work in, and activate it.

  2. Clone the repository:

    $ git clone https://github.com/nedbat/coveragepy
$ cd coveragepy

  3. Install the requirements:

    $ pip install -r requirements/dev.pip

  4. Install a number of versions of Python. Coverage.py supports a range of Python versions. The more you can test with, the more easily your code can be used as-is. If you only have one version, that’s OK too, but may mean more work integrating your contribution.

Running the tests

The tests are written mostly as standard unittest-style tests, and are run with pytest running under tox:

$ tox
py37 create: /Users/nedbat/coverage/trunk/.tox/py37
py37 installdeps: -rrequirements/pip.pip, -rrequirements/pytest.pip, eventlet==0.25.1, greenlet==0.4.15
py37 develop-inst: /Users/nedbat/coverage/trunk
py37 installed: apipkg==1.5,appdirs==1.4.4,attrs==20.3.0,backports.functools-lru-cache==1.6.4,-e git+git@github.com:nedbat/coveragepy.git@36ef0e03c0439159c2245d38de70734fa08cddb4#egg=coverage,decorator==5.0.7,distlib==0.3.1,dnspython==2.1.0,eventlet==0.25.1,execnet==1.8.0,filelock==3.0.12,flaky==3.7.0,future==0.18.2,greenlet==0.4.15,hypothesis==6.10.1,importlib-metadata==4.0.1,iniconfig==1.1.1,monotonic==1.6,packaging==20.9,pluggy==0.13.1,py==1.10.0,PyContracts @ git+https://github.com/slorg1/contracts@c5a6da27d4dc9985f68e574d20d86000880919c3,pyparsing==2.4.7,pytest==6.2.3,pytest-forked==1.3.0,pytest-xdist==2.2.1,qualname==0.1.0,six==1.15.0,sortedcontainers==2.3.0,toml==0.10.2,typing-extensions==3.10.0.0,virtualenv==20.4.4,zipp==3.4.1
py37 run-test-pre: PYTHONHASHSEED='376882681'
py37 run-test: commands[0] | python setup.py --quiet clean develop
py37 run-test: commands[1] | python igor.py zip_mods remove_extension
py37 run-test: commands[2] | python igor.py test_with_tracer py
=== CPython 3.7.10 with Python tracer (.tox/py37/bin/python) ===
bringing up nodes...
........................................................................................................................................................... [ 15%]
........................................................................................................................................................... [ 31%]
...........................................................................................................................................s............... [ 47%]
...........................................s...................................................................................sss.sssssssssssssssssss..... [ 63%]
........................................................................................................................................................s.. [ 79%]
......................................s..................................s................................................................................. [ 95%]
........................................ss......                                                                                                            [100%]
949 passed, 29 skipped in 40.56s
py37 run-test: commands[3] | python setup.py --quiet build_ext --inplace
py37 run-test: commands[4] | python igor.py test_with_tracer c
=== CPython 3.7.10 with C tracer (.tox/py37/bin/python) ===
bringing up nodes...
........................................................................................................................................................... [ 15%]
........................................................................................................................................................... [ 31%]
......................................................................s.................................................................................... [ 47%]
........................................................................................................................................................... [ 63%]
..........................s................................................s............................................................................... [ 79%]
.................................................................................s......................................................................... [ 95%]
......................................s.........                                                                                                            [100%]
973 passed, 5 skipped in 41.36s
____________________________________________________________________________ summary _____________________________________________________________________________
  py37: commands succeeded
  congratulations :)

Tox runs the complete test suite twice for each version of Python you have installed. The first run uses the Python implementation of the trace function, the second uses the C implementation.

To limit tox to just a few versions of Python, use the -e switch:

$ tox -e py37,py39

To run just a few tests, you can use pytest test selectors:

$ tox tests/test_misc.py
$ tox tests/test_misc.py::HasherTest
$ tox tests/test_misc.py::HasherTest::test_string_hashing

These command run the tests in one file, one class, and just one test, respectively.

You can also affect the test runs with environment variables. Define any of these as 1 to use them:

  • COVERAGE_NO_PYTRACER=1 disables the Python tracer if you only want to run the CTracer tests.

  • COVERAGE_NO_CTRACER=1 disables the C tracer if you only want to run the PyTracer tests.

  • COVERAGE_ONE_TRACER=1 will use only one tracer for each Python version. This will use the C tracer if it is available, or the Python tracer if not.

  • COVERAGE_AST_DUMP=1 will dump the AST tree as it is being used during code parsing.

There are other environment variables that affect tests. I use set_env.py as a simple terminal interface to see and set them.

Of course, run all the tests on every version of Python you have, before submitting a change.

Lint, etc

I try to keep the coverage.py source as clean as possible. I use pylint to alert me to possible problems:

$ make lint

The source is pylint-clean, even if it’s because there are pragmas quieting some warnings. Please try to keep it that way, but don’t let pylint warnings keep you from sending patches. I can clean them up.

Lines should be kept to a 100-character maximum length. I recommend an editorconfig.org plugin for your editor of choice.

Other style questions are best answered by looking at the existing code. Formatting of docstrings, comments, long lines, and so on, should match the code that already exists.

Many people love black, but I would prefer not to run it on coverage.py.

Continuous integration

When you make a pull request, GitHub actions will run all of the tests and quality checks on your changes. If any fail, either fix them or ask for help.

Dependencies

Coverage.py has no direct runtime dependencies, and I would like to keep it that way.

It has many development dependencies. These are specified generically in the requirements/*.in files. The .in files should have no versions specified in them. The specific versions to use are pinned in requirements/*.pip files. These are created by running make upgrade.

It’s important to use Python 3.7 to run make upgrade so that the pinned versions will work on all of the Python versions currently supported by coverage.py.

If for some reason we need to constrain a version of a dependency, the constraint should be specified in the requirements/pins.pip file, with a detailed reason for the pin.

Coverage testing coverage.py

Coverage.py can measure itself, but it’s complicated. The process has been packaged up to make it easier:

$ make metacov metahtml

Then look at htmlcov/index.html. Note that due to the recursive nature of coverage.py measuring itself, there are some parts of the code that will never appear as covered, even though they are executed.

Contributing

When you are ready to contribute a change, any way you can get it to me is probably fine. A pull request on GitHub is great, but a simple diff or patch works too.

All contributions are expected to include tests for new functionality and fixes. If you need help writing tests, please ask.