For plugin development, the docs say to use

interpreter_constraints=["==3.9.*"]

because 3.9 is what pants internally uses (and 3.9 is different from the version we use, like in the example in the docs). https://www.pantsbuild.org/2.21/docs/writing-plugins/overview#building-in-repo-plugins-with-pants When we make

python_source

entries for the plugin files (to get code formatting, linting, etc.) on the in-repo plugin code, we now need python 3.9 for development (in addition to the version we otherwise use). Is there a way to make pants use the python installation for plugin development that comes with pants itself and is used by it for the plugin execution? Otherwise, users now would need to install two python versions on their system, 3.9 and whatever we use use, e.g. 3.10 (in addition to the 3.9 that comes with pants).

It is a reasonable request! But yes that is the current state of affairs.