example-python

An example repository to demonstrate Python support in Pants.

See pantsbuild.org for much more detailed documentation.

This is only one possible way of laying out your project with Pants. See pantsbuild.org/docs/source-roots#examples for some other example layouts.

Running Pants

You run Pants goals using the pants launcher binary, which will bootstrap the version of Pants configured for this repo if necessary.

See here for how to install the pants binary.

❓ Running with Apple Silicon and/or macOS? You will want to make changes to the search_path and interpreter_constraints values in pants.toml before running pants - there is guidance in pants.toml for those settings.

Use pants --version to see the version of Pants configured for the repo (which you can also find in pants.toml).

Goals

Pants commands are called goals. You can get a list of goals with

pants help goals

Targets

Targets are a way of setting metadata for some part of your code, such as timeouts for tests and entry points for binaries. Targets have types like python_source, resources, and pex_binary. They are defined in BUILD files.

Pants goals can be invoked on targets or directly on source files (which is often more intuitive and convenient). In the latter case, Pants locates target metadata for the source files as needed.

File specifications

Invoking goals on files is straightforward, e.g.,

pants test helloworld/greet/greeting_test.py

You can use globs:

pants lint helloworld/greet/*.py

But note that these will be expanded by your shell, so this is equivalent to having used

pants lint helloworld/greet/__init__.py helloworld/greet/greeting.py helloworld/greet/greeting_test.py

If you want Pants itself to expand the globs (which is sometimes necessary), you must quote them in the shell:

pants lint 'helloworld/greet/*.py'

You can run on all changed files:

pants --changed-since=HEAD lint

You can run on all changed files, and any of their "dependents":

pants --changed-since=HEAD --changed-dependents=transitive test

Target specifications

Targets are referenced on the command line using their address, of the form path/to/dir:name, e.g.,

pants lint helloworld/greet:lib

You can glob over all targets in a directory with a single trailing :, or over all targets in a directory and all its subdirectories with a double trailing ::, e.g.,

pants lint helloworld::

Globbing semantics

When you glob over files or targets, Pants knows to ignore ones that aren't relevant to the requested goal. For example, if you run the test goal over a set of files that includes non-test files, Pants will just ignore those, rather than error. So you can safely do things like

pants test ::

To run all tests.

Example Goals

Try these out in this repo!

List targets

pants list ::  # All targets.
pants list 'helloworld/**/*.py'  # Just targets containing Python code.

Run linters and formatters

pants lint ::
pants fmt helloworld/greet::

Run MyPy

pants check ::

Run tests

pants test ::  # Run all tests in the repo.
pants test --output=all ::  # Run all tests in the repo and view pytest output even for tests that passed (you can set this permanently in pants.toml).
pants test helloworld/translator:tests  # Run all the tests in this target.
pants test helloworld/translator/translator_test.py  # Run just the tests in this file.
pants test helloworld/translator/translator_test.py -- -k test_unknown_phrase  # Run just this one test by passing through pytest args.

Create a PEX binary

The package goal requires specifying a target which can be packaged. In this case, the there is a pex_binary target with the name pex_binary in the helloworld/BUILD file.

pants package helloworld:pex_binary

The pex file is output to dist/helloworld/pex_binary.pex and can be executed directly.

Run a binary directly

pants run helloworld/main.py

Open a REPL

pants repl helloworld/greet:lib  # The REPL will have all relevant code and dependencies on its sys.path.
pants repl --shell=ipython helloworld/greet:lib --no-pantsd  # To use IPython, you must disable Pantsd for now.

Build a wheel / generate setup.py

This will build both a .whl bdist and a .tar.gz sdist.

pants package helloworld/translator:dist

Count lines of code

pants count-loc '**/*'

Generate or update a lockfile containing the dependencies

pants generate-lockfiles --resolve=python-default

Create virtualenv for IDE integration

pants export --resolve=python-default