pip_parse vendored

This example is like pip_parse, however we avoid loading from the generated file. See https://github.com/bazelbuild/rules_python/issues/608 and https://blog.aspect.dev/avoid-eager-fetches.

The requirements now form a triple:

• requirements.in - human editable, expresses only direct dependencies and load-bearing version constraints
• requirements.txt - lockfile produced by pip-compile or other means
• requirements.bzl - the “parsed” version of the lockfile readable by Bazel downloader

The requirements.bzl file contains baked-in attributes such as python_interpreter_target as they were specified in the original pip_parse rule. These can be overridden at install time by passing arguments to install_deps. For example:

# Register a hermetic toolchain
load("@rules_python//python:repositories.bzl", "python_register_toolchains")

python_register_toolchains(
    name = "python39",
    python_version = "3.9",
)
load("@python39//:defs.bzl", "interpreter")

# Load dependencies vendored by some other ruleset.
load("@some_rules//:py_deps.bzl", "install_deps")

install_deps(
    python_interpreter_target = interpreter,
)