Fix conda executable not found (conda 4.4+)

[benbovy]

Fixes #73

Try first getting path to conda executable from $CONDA_EXE.

xref: jupyterlab/jupyterlab#4598

[coveralls]

Coverage increased (+0.4%) to 48.364% when pulling de9f8ac on benbovy:conda44-fix into f7b4680 on Anaconda-Platform:master.

[tjd2002]

Thanks @benbovy this allows me to use nb_conda under latest anaconda (4.5.3)!

It seems this repo needs a little attention to catch up to the changes in 4.4. Are there any maintainers around? @damianavila or @bollwyvl perhaps ?

[tjd2002]

Looks like this will not be compatible with older installs of conda that do not export $CONDA_EXE. Is this a problem? See #80 for a PR that includes a backwards-compatible fix (among other changes)

[mcg1969]

To be honest, #80 is getting too unwieldy, trying to do too many things at once. It's fine with me if we try to separate concerns here. I would propose that you bring in my back-compatible logic (or equivalent) into your construction of CONDA_EXE, but that this PR be treated/reviewed/merged separately.

[benbovy]

Looks like this will not be compatible with older installs of conda that do not export $CONDA_EXE. Is this a problem? See #80 for a PR that includes a backwards-compatible fix (among other changes)

In this PR, if $CONDA_EXE doen't exists then it fails back to the older behavior, i.e., trying to get conda from PATH, so I think it is still backward compatible.

[benbovy]

That said, #80 seems to test for a few more things, notably on windows platforms which I don't use, so it's probably better addressed there.

[mcg1969]

I agree now that the approach in #80 can at least be simplified. For #82, I've opted to pull this in directly. If #80 needs something more, it will get it ;-)

[tjd2002]

As to whether the special-casing in #80 is necessary,

At https://github.com/Anaconda-Platform/anaconda-nb-extensions/issues/168#issuecomment-392280875, Kale Franz wrote:

I would wait to target any change toward conda 4.6+. [...] The big differences here are the existence of a new conda init command to ensure your shell is configured correctly, and then the guaranteed presence of a CONDA_EXE environment variable (and CONDA_BAT as appropriate).

(This: conda/conda#7206 appears to be the PR that makes these changes in 4.6)

So I agree with @mcg1969 that the approach used here looks good.

I think it is preferable to consider this separately from #82, since it is a simple fix for an issue that is happening in the wild right now (for users trying out the new-style install).

[mcg1969]

Good point, I would definitely support merging this first.

[mcg1969]

Based on discussions with @tjd2002, this is still included in #82, so this can be closed (and #73) if #82 is merged.

[benbovy]

OK, I leave this PR as is for now. Let me know when I can close this (or for maintainers, feel free to close it or merge it depending on what's best to do).

[mcg1969]

I'm still waiting on some internal decision making and @damianavila in particular for advice on how to proceed. Stay tuned. Won't be too much longer, I don't think.

[mcg1969]

Addressed by #82.