I am trying to change the paraview package so that version 5.6 uses python3 and all previous version use python2. So I have the following code:

depends_on('python@3:', when='@5.6:+python', type=('build', 'link', 'run'))
depends_on('python@2.7:2.8', when='@:5.5+python', type=('build', 'link', 'run'))

If I don't do the same branching on all python packages, I will get the error in the title for the following command:

spack spec paraview+python

However, if I specify the version that the previous command chooses, the command works correctly.

spack spec paraview@5.6.0+python

Numpy is one of the packages that are needed in paraview. I will have to use:

depends_on('py-numpy', when='@:5.5+python', type=('build', 'run'))
depends_on('py-numpy', when='@5.6:+python', type=('build', 'run'))

instead of the simpler:

depends_on('py-numpy', when='+python', type=('build', 'run'))

to avoid the error in the title. Attached is the package.py that shows this error.
paraview.zip