[!18] Make iteration order of versions deterministic to ensure default version

Merge branch 'feature/deterministic-version' into 'master'

See merge request [spack/dune-spack!18]

  [spack/dune-spack!18]: Nonespack/dune-spack/merge_requests/18

packages/dune/package.py

@@ -33,11 +33,13 @@ class Dune(CMakePackage):
     git = "https://gitlab.dune-project.org/core/dune-common.git"
 
     # This defines a mapping of available versions of the dune Spack package
-    # and the branch name in the Dune repositories this refers to.
-    dune_versions_to_branch = {
-      "master" : "master",
-      "2.7" : "releases/2.7",
-    }
+    # and the branch name in the Dune repositories this refers to. This is a
+    # list instead of a dictionary to ensure iteration order (first entry is
+    # the default version) in Python3.
+    dune_versions_to_branch = [
+      ("2.7", "releases/2.7"),
+      ("master" , "master"),
+    ]
 
     # Variants for the general build process
     variant('shared', default=True, description='Enables the build of shared libraries.')
@@ -89,7 +91,7 @@ class Dune(CMakePackage):
     # Iterate over all available Dune versions and define resources for all Dune modules
     # If a Dune module behaves differently for different versions (e.g. dune-python got
     # merged into dune-common post-2.7), define the resource outside of this loop.
-    for vers, branch in dune_versions_to_branch.items():
+    for vers, branch in dune_versions_to_branch:
         version(vers, branch=branch)
 
         resource(