Wondering how this affects concretization since Python 2 is the default in Spack.

You have to explicitly specify ^[email protected]:, otherwise you will get:

py-matplotlib requires python version 3.5:, but spec asked for 2.7.15

Could we perhaps allow a depends_on to override the "preferred=True"?

Could we perhaps allow a depends_on to override the "preferred=True"?

That would definitely be the preferred behavior. Not sure how much work would be required to make that happen. @tgamblin @scheibelp?

Any thoughts on when we want to make Python 3 the default? How many Python 2-only packages remain?

We should really be able to have packages depend on python 2/3/either when at different versions. Lemme think about this one. Is there a decent workaround you could use for right now?

Maybe we could remove the preferred=True from [email protected] and instead ship a packages_python27.yaml, which could take everything that is incompatible with py3 (but we will probably add to this over the next month or two)

I don't think we've come to any conclusion on this one. Could we merge it as is with the caveat that users need to explicitly specify ^[email protected]: and then work on a solution in a separate issue/PR?

Could we merge it as is with the caveat that users need to explicitly specify ^[email protected]: and then work on a solution in a separate issue/PR?

Agreed - thanks!