resurrect preferred=True option for packages lost in merge of externals support.

- Pyton 2.7.11 is preferred again.

lib/spack/spack/concretize.py

@@ -159,6 +159,10 @@ def concretize_version(self, spec):
              if any(v.satisfies(sv) for sv in spec.versions)],
             cmp=cmp_versions)
 
+        def prefer_key(v):
+            return pkg.versions.get(Version(v)).get('preferred', False)
+        valid_versions.sort(key=prefer_key, reverse=True)
+
         if valid_versions:
             spec.versions = ver([valid_versions[0]])
         else:

lib/spack/spack/preferred_packages.py

@@ -150,9 +150,7 @@ def spec_has_preferred_provider(self, pkgname, provider_str):
     def version_compare(self, pkgname, a, b):
         """Return less-than-0, 0, or greater than 0 if version a of pkgname is
            respecively less-than, equal-to, or greater-than version b of pkgname.
-           Versions marked 'preferred=True' in package.py take precedence over any
-           versions not marked preferred.
-        """
+           One version is less-than another if it is preferred over the other."""
         return self._spec_compare(pkgname, 'version', a, b, True, None)
 
 

lib/spack/spack/test/concretize.py

@@ -24,6 +24,7 @@
 ##############################################################################
 import spack
 from spack.spec import Spec, CompilerSpec
+from spack.version import ver
 from spack.concretize import find_spec
 from spack.test.mock_packages_test import *
 
@@ -77,6 +78,14 @@ def test_concretize_variant(self):
         self.check_concretize('mpich')
 
 
+    def test_concretize_preferred_version(self):
+        spec = self.check_concretize('python')
+        self.assertEqual(spec.versions, ver('2.7.11'))
+
+        spec = self.check_concretize('python@3.5.1')
+        self.assertEqual(spec.versions, ver('3.5.1'))
+
+
     def test_concretize_with_virtual(self):
         self.check_concretize('mpileaks ^mpi')
         self.check_concretize('mpileaks ^mpi@:1.1')