Hatch doesn't pick up python version when changed in the config

[nikitajz]

Issue

When tool.hatch.envs.default.python is specified after the first run (was not before) or amended, hatch keeps using the old version of python. In order to actually apply the change, the old environment should be deleted.
It might not be interpreted as a bug per ce, but it's not what is expected intuitively.

How to reproduce

Stage 1: No python version is specified under tool.hatch.*

pyproject.toml:

[project]
requires-python = ">=3.9"
# ...

[tool.hatch.envs.default]
extra-dependencies = [
    "pytest~=7.4.4"
]

Run: hatch run pytest
Hatch installs the latest available python version, for example, python=3.12

❯ hatch run pytest
============================================ test session starts ============================================
platform darwin -- Python 3.12.2, pytest-7.4.4, pluggy-1.4.0
...

Stage 2: Specify python version explicitly

pyproject.toml:

# ...

[tool.hatch.envs.default]
python=3.9

#...

Run: hatch run test
Expected: Hatch rebuild environment using python=3.9
Actual: Hatch keeps using python=3.12

To use python=3.9 you have to remove existing env manually, for example, hatch env prune

[nikitajz]

Seems loosely related #1097 and #1126

[a-reich]

+1 to this - it tripped me up because the docs say “dependencies are automatically synced” each time the environment is used, but it didn’t occur to me that this would mean only package dependencies and not python version.