conda: use an environment file

[henryiii]

How would this feature be useful?
Many projects define dependencies in an environment file. You can list channels, etc. in such a file. For example:

name: myenv
channels:
  - conda-forge
dependencies:
  - python==3.9
  - pip >=18
  - pytest >=5
  - pip:
    - -e .

More realistic example here: https://gitlab.cern.ch/LHCb-Reco-Dev/pv-finder/-/blob/master/environment.yml

I don't see a way to specify this with conda_install, since it's a different command used when making the environment (and it can set a Python version for the conda environment).

Describe the solution you'd like

Open for ideas. It seems rather like it should be part of the session decorator? But not sure how it might tie in. One possible idea would be to pick up environment.yml if it exists (conda env create will pick it up that name automatically), but there should be a way to configure it (-f environement2-yml, etc) or ignore an environement.yml if one exists)?

Currently I want to set up an environment for nox, but in GHA, I'd like to use https://github.com/conda-incubator/setup-miniconda - so this would let me share the (non-trivial) environment between nox, GHA, and mybinder (which automatically loads an environment.yml file).

[henryiii]

FYI, it's not as "nice", but you can use conda update to inject an environment into the current environment; this is how mybinder was implemented, IIRC. You can't keep the environment name this way, which can be affect running notebook kernels.

[henryiii]

Oops, dup of #260

[cjolowicz]

Thanks @henryiii , closing as duplicate.