rules_pycross - Python + cross platform

A proof-of-concept ruleset to (hopefully) improve cross-platform Python builds with Bazel.

Note: very much still experimental.

Features:

• A single lock file for all target platforms, thanks to Poetry
• Builds that happen in build actions, not during WORKSPACE initialization
• Standard Bazel http_file rules used for fetching dependencies. pip is not a build-time dependency.

See the example.

Why?

The current Bazel rules for working with Python external dependencies have a couple of issues that make cross-platform usage difficult (see bazel-contrib/rules_python#260):

• they're based on pip and pip-compile which do not generate cross-platform lock files. For example, IPython depends on appnope only on MacOS. Lock files generated by pip-compile will differ based on whether they're created on Linux or MacOS.
• They use pip install during the WORKSPACE phase to fetch and possibly build packages (including native libraries). WORKSPACE operations lack many of the things that Bazel's build actions provide such as sandboxing and remote execution.

How?

A pip install operation can be roughly broken down into these parts:

1. determine the environment (OS and Python version/implementation)
2. resolve the dependencies of the package to install, some of which may be platform-specific (optionally constrained by a pre-compiled lock file)
3. figure out which files to download - either pre-built wheels matching the current platform or sdists to build locally
4. download sdists and wheels
5. build and install sdists; install wheels

rules_pycross attemps to deconstruct this operation into its constituent parts and glue them together with Bazel:

1. pycross_target_environment is used to specify target environments ahead of time provided with ABI, platform, and implementation parameters (similar to pip's --abi, --platform, and --implementation flags). These environments are selected using Bazel's own platform/constraint system.
2. pycross_lock_file generates a "lock" .bzl file from an input poetry.lock. This .bzl file contains a mix of http_file repositories and pycross_* targets.
3. pycross_wheel_build builds sdist.tar.gz archives into Python wheels. This is a build action, not a WORKSPACE operation.
4. pycross_wheel_library "installs" (extracts) a Python wheel - either downloaded or built from an sdist - and provides it as a py_library.

See the generated docs.