Provide a way to "conda env update" an environment: or more general conda commands.

[mforbes]

How would this feature be useful?
Another common way of provisioning conda environments is through an environment.yml file which can be used to update an environment with

conda env update --file environment.yml

Describe the solution you'd like
Not sure what is ideal here. This particular use-case could be solved by adding a command:

    session.conda_env_update("environment.yml")

or similar. This is easily implemented in session.py:

   ...
    def conda_env_update(self, envfile='environment.yml': str, **kwargs: Any) -> None:
        """Install invokes `conda env update`_ to update the session's
        environment.

        To install packages directly::

            session.conda_env_update('environment.yml')

        .. _conda env update:
        """
        venv = self._runner.venv

        prefix_args = ()  # type: Tuple[str, ...]
        if isinstance(venv, CondaEnv):
            prefix_args = ("--prefix", venv.location)
        elif not isinstance(venv, PassthroughEnv):  # pragma: no cover
            raise ValueError(
                "A session without a conda environment cannot update."
            )

        if "silent" not in kwargs:
            kwargs["silent"] = True

        self._run(
            "conda", "env", "update", *prefix_args, "--file", envfile, external="error", **kwargs
        )
   ...

however, it might be better to provide a more general fallback that allows users to customize commands for conda provisioning.

    session.conda("env update --file environment.yml")

possibly with variables for the --prefix argument. The other commands like conda_install could then be aliases to this more generic command. Note: there are some differences between conda install and conda env command (for example, --yes is not supported by the latter) which can cause confusion.

Related: one might also like to replace conda with mamba, so perhaps a kwarg could be provided. I will play with this a bit and try to make a reasonable PR/

[smarie]

Looks interesting @mforbes ! Could this be a possible duplicate of #260 ?

[mforbes]

Yes - it looks very similar. Sorry, not sure how I missed that... I will review what was discussed there, then merge my comments and discussion with that issue and then close this.

[bjlittle]

@mforbes and @smarie I've just discovered nox, and it's really pretty awesome! Congratulations 💯 🎉

I'd love to be able to create a conda environment based on an environment.yml so it would be great to know if this issue will mature into a pull-request?

[theacodes]

Closing in favor of #260