How to share Python code between multiple packages without needing to publish the code as a library

[gaborbernat]

So this question is a variation of the monorepo support. Imagine you have a project that generates 2 or more (e.g. A, B) distinct maturin projects. However, there's some common code you want to share between them:

.
├──  Cargo.lock
├──  Cargo.toml
├── rust-common
│   ├──  Cargo.toml
│   ├──  src
│   └── target
├── py-common
│   ├──  CHANGELOG.md
│   ├──  pyproject.toml
│   └──  src
├── A
│   ├──  Cargo.toml
│   ├──  pyproject.toml
│   ├── rust
│   └── src
├── B
│   ├──  Cargo.toml
│   ├──  pyproject.toml
│   ├── rust
│   └── src
└──  rust-toolchain.toml

Sharing the rust code (hosted here in rust-common without needing to publish the crate is possible via cargo allowing relative paths for dependencies and rust has a virtual workspace feature. The question is: How can you share the Python code? The Python part of PEP-621 does not allow relative paths in dependencies.

Ideally, I would be able to take the content of src from py-common and mount it (copy it, symlink it) under src/a/py-common. Something like:

[tool.maturin]
python-dependency-local = ['../py-common']

Which would then cause:

• sdist (no op?)
• wheel - build a wheel of py-common, take the non dist-info part, and copy it within the projects package,
• editable - mark the project as an editable dependency.

Things I have tried:

• using python-packages with a relative path, but this does not make sense because is unclear where it should live.
• creating symlink (maturin seems to ignore the symlink part of the wheel build (but not sdist)).

Example project tox-dev/toml-fmt#12, in this case trying to share py-common under pyproject-fmt, without needing to publish the py-common to PyPI.