Commits on Sep 4, 2024

1. Add initial support for Poetry …

   The Python package manager Poetry is now supported for installing app
   dependencies:
   https://python-poetry.org
   
   To use Poetry, apps must have a `poetry.lock` lockfile, which can be
   created by running `poetry lock` locally, after adding Poetry config to
   `pyproject.toml` (which can be done either manually or by using
   `poetry init`). Apps must only have one package manager file (either
   `requirements.txt` or `poetry.lock`, but not both) otherwise the
   buildpack will abort the build with an error (which will help prevent
   some of the types of support tickets we see in the classic buildpack
   with users unknowingly mixing and matching pip + Pipenv).
   
   Poetry is installed into a build-only layer (to reduce the final app
   image size), so is not available at run-time. The app dependencies are
   installed into a virtual environment (the same as for pip after #257,
   for the reasons described in #253), which is on `PATH` so does not need
   explicit activation when using the app image. As such, use of
   `poetry run` or `poetry shell` is not required at run-time to use
   dependencies in the environment.
   
   When using Poetry, pip is not installed (possible thanks to #258), since
   Poetry includes its own internal vendored copy that it will use instead
   (for the small number of Poetry operations for which it still calls out
   to pip, such as package uninstalls).
   
   Both the Poetry and app dependencies layers are cached, however, the
   Poetry download/wheel cache is not cached, since using it is slower than
   caching the dependencies layer (for more details see the comments on
   `poetry_dependencies::install_dependencies`).
   
   The `poetry install --sync` command is run using `--only main` so as to
   only install the main `[tool.poetry.dependencies]` dependencies group
   from `pyproject.toml`, and not any of the app's other dependency groups
   (such as test/dev groups, eg `[tool.poetry.group.test.dependencies]`).
   
   I've marked this `semver: major` since in the (probably unlikely) event
   there are any early-adopter projects using this CNB that have both a
   `requirements.txt` and `poetry.lock` then this change will cause them to
   error (until one of the files is deleted).
   
   Relevant Poetry docs:
   - https://python-poetry.org/docs/cli/#install
   - https://python-poetry.org/docs/configuration/
   - https://python-poetry.org/docs/managing-dependencies/#dependency-groups
   
   Work that will be handled later:
   - Support for selecting Python version via `tool.poetry.dependencies.python`:
     #260
   - Build output and error messages polish/CX review (this will be performed
     when switching the buildpack to the new logging style).
   - More detailed user-facing docs:
     #11
   
   Closes #7.
   GUS-W-9607867.
   GUS-W-9608286.
   GUS-W-9608295.

   

   edmorley committed Sep 4, 2024
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