Try the following:

1. Clone latest Spack develop.
2. Use the following packages.yaml:

packages:
    python:
        version: [3.5]

1. spack spec python yields:

  python@2.7.12%gcc@4.8.5~tk~ucs4 arch=linux-centos7-x86_64

This is wrong. preferred=True should have the same precedence as one version simply having the highest number. It is possible to get v3.5 with spack spec python@3:

python@3.5.2%gcc@4.8.5~tk~ucs4 arch=linux-centos7-x86_64

For now, I'm removing preferred=True from python/package.py.