RFD 1135: One Python environment per Elixir app

Problem

service-livebook embeds a Python interpreter with pythonx. Its setup cell names the packages and the pins travel in the notebook file. Beside that, a helper shelled into five environments in another repository. The notebook described three packages while the loop depended on five environments it never names, so it ran on one desk and nowhere else.

The failure was not theoretical. Loop 1 stopped with No module named 'drjit', because it called the anny environment for a renderer that only the default environment carries. The notebook cannot show that.

Decision

An Elixir app reaches Python through pythonx and through nothing else. priv/python/check_pixi_free.py fails the build on a call to the other environment manager, and it reads prose without complaining, so the rule stays writable.

An embedded interpreter starts once, with one dependency set. So a second dependency set is a second app: its own mix project, its own setup cell, its own runtime. The gate holds that too. It reads the pyproject.toml cell of every notebook in an app and fails when two of them differ.

This is what the corpus repository’s environments were for. OmniGen2 pins torch 2.6.0+cu124 and EditScore pins cu128, and no interpreter holds both. That separation stays, as apps beside each other rather than environments inside one app. The corpus repository keeps its own environments, and what is forbidden is this app calling into them.

Related

RFD 1134 tests these notebooks in the browser, which is how the missing renderer was found.