Introduce NonVersionFolderName for more clarity

src/docs_versions_menu/groups.py

@@ -1,6 +1,6 @@
 """Classification of folders into groups according to :pep:`440`."""
 
-from .parse_version import LegacyVersion, parse_version
+from .parse_version import NonVersionFolderName, parse_version
 
 
 def get_groups(folders, default_branches=None):
@@ -51,7 +51,7 @@ def get_groups(folders, default_branches=None):
         version = parse_version(folder)
         if folder in default_branches:
             groups['default-branch'].add(folder)
-        if isinstance(version, LegacyVersion):
+        if isinstance(version, NonVersionFolderName):
             groups['branches'].add(folder)
         else:
             groups['releases'].add(folder)

src/docs_versions_menu/parse_version.py

@@ -8,9 +8,19 @@
 import packvers.version
 
 
-LegacyVersion = packvers.version.LegacyVersion
+class NonVersionFolderName(packvers.version.LegacyVersion):
+    """A "version" that is just an arbitrary folder name"""
+
+    # We rely on the sorting properties of LegacyVersion relative to to a
+    # proper Version, even though arbitrary folder names aren't in any sense
+    # "legacy versions"
+    pass
 
 
 def parse_version(version):
-    """Parse `version` string into an object that allows version comparisons."""
-    return packvers.version.parse(version)
+    """Parse `version` string into either a `Version` or a
+    `NonVersionFolderName` object."""
+    try:
+        return packvers.version.Version(version)
+    except packvers.version.InvalidVersion:
+        return NonVersionFolderName(version)