I'm creating a series of packages to get jupyter notebooks working in spack.

One such package is python tornado.

I've created this package and added the line:

depends_on('py-backports_abc', when="^python@:3", type="nolink")

among others.

I've created the package py-backports_abc as well. When I try to install py-tornado, I get the following baffling error message:

spack install py-tornado ^python@2.7.12
==> Error: py-tornado does not depend on py-backports_abc

I don't understand why spack is giving this error.