Autogen interpreter script uses #!/usr/bin/env python3 instead of meson's python, breaking builds where multiple python3s exist on PATH #41895
Description
Hello; I have wanted to build sage from source on NixOS for awhile now. I persuaded a Claude to get it working (it succeeded!), and it requested I open an issue regarding the following. I've checked the literal claims below, and I think it's correct, but I am not an expert. (I suspect this is a separate issue; but mesonbuild/meson#7372 is referenced in the relevant meson.build file, and it is now marked as 'Not planned'.)
In src/sage/ext/interpreters/meson.build, the custom command at line 27 invokes main.py directly:
command: ['../../../sage_setup/autogen/interpreters/__main__.py', '@OUTDIR@'],
This script has #!/usr/bin/env python3, so it runs whichever python3 the OS finds first on PATH. When building in environments like NixOS where multiple python3 interpreters exist (some without build dependencies like jinja2; these will almost always appear because nix will create other python's for other build dependencies, for eg, pkgs.meson), this fails with ModuleNotFoundError even though meson-python's own python has all required packages.
The fix would be to use meson's python interpreter explicitly:
command: [py, '../../../sage_setup/autogen/interpreters/__main__.py', '@OUTDIR@'],
where py is the python found by meson (already used elsewhere in the build and this particular custom_target call). This ensures the script runs with the same interpreter that has all build dependencies.
Workaround: Setting PYTHONPATH to the wrapped python's site-packages in the shell environment.