in #12890 we synchronized (core) dependencies listed in our README with those in pyproject.toml. I think we should do the same with environment.yaml too. I wrote a quick script to compare them:

import toml
import yaml

pyproj = toml.load("pyproject.toml")
# ignore "full" as it's just "full-noqt" plus PyQt6, and for conda we need PySide
ignore = ("dev", "doc", "test", "test_extra", "full", "full-pyqt6")
deps = set(pyproj["project"]["dependencies"])
for section, _deps in pyproj["project"]["optional-dependencies"].items():
    if section not in ignore:
        deps |= set(_deps)
recursive_deps = set(x for x in deps if x.startswith("mne["))
deps -= recursive_deps

with open("environment.yml") as fid:
    deps_env = set(yaml.safe_load(fid)["dependencies"])

print(sorted(deps - deps_env))
print()
print(sorted(deps_env - deps))

Here's the differences:

Seems like it would be fairly easy to add the deps that are missing from TOML, then generate environment.yaml dynamically like we do the README (in a CI hook that listens for pyproject.toml changes):

• The list of things that we want in environment.yaml that can't go in the TOML file is pretty short (mne-base, mamba, openblas, pip, spyder-kernels), and could go in a small file in the repo root (.conda-env-extras.yaml or so)
• I think we could nix mne-base --- what we spec in pyproject.toml should encompass everything in mne-base (and if it doesn't, we should fix that!). Not saying we should stop providing mne-base, just that we shouldn't need it to be a dep in our main environment.yaml file.
• I think we could nix mamba nowadays (libmamba solver is default for a while now).