rules_venv

This repository implements Bazel rules for Python and is designed to be a drop-in replacement for the existing rules_python where uses of @rules_python//python:defs.bzl can be replaced with @rules_venv//python:defs.bzl.

Improvements over rules_python

While rules_python has fantastic toolchain infrastructure which this repo relies on, rules_python ultimately suffers from a few issues which this repo aims to solve:

1. Use of PYTHONPATH to construct the python environment leads to operating system limitations.

   Some details on MAX_ARG_STRLEN and ARG_MAX can be found here: https://unix.stackexchange.com/a/120842

2. Slow startup on windows systems that do not support symlinks.

   rules_python creates zipapps on systems that do not support runfiles. For large projects, this can lead to large (~500MB+) zip files being constantly compressed and uncompressed to run simple actions which is a lot more expensive than systems which support runfiles.

Setup

bazel_dep(name = "rules_venv", version = "{version}")

Environment variables

RULES_VENV_EXTRACT_ROOT

By default each launch of a py_binary or py_test builds its venv in a temporary directory that is deleted on exit, so sys.executable is only valid while the process runs. When this variable is set, the venv is instead built at $RULES_VENV_EXTRACT_ROOT/<repo>/<package>/<name>.venv (with _main for the main repository) and, on platforms without runfiles support, the rendered runfiles tree beside it as <name>.runfiles. Both are left in place and reused by later launches of the same target unless the interpreter, import paths, or runfiles collection have changed. Cleaning up the root is the responsibility of the caller. A relative path is resolved against the working directory.

RULES_PYTHON_EXTRACT_ROOT

Honored like RULES_VENV_EXTRACT_ROOT so a root configured for rules_python is shared, but RULES_VENV_EXTRACT_ROOT takes priority when both are set.