Add an API to describe variant using labels

tests/test_plugins.py

@@ -5,6 +5,7 @@
 
 from variantlib.base import PluginBase
 from variantlib.config import KeyConfig, ProviderConfig
+from variantlib.meta import VariantDescription, VariantMeta
 from variantlib.plugins import PluginLoader
 
 
@@ -20,6 +21,12 @@ def get_supported_configs(self) -> Optional[ProviderConfig]:
             ],
         )
 
+    def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
+        for meta in variant_desc:
+            if meta.namespace == self.namespace and meta.key == "key1":
+                return [meta.value.removeprefix("val")]
+        return []
+
 
 # NB: this plugin deliberately does not inherit from PluginBase
 # to test that we don't rely on that inheritance
@@ -34,9 +41,28 @@ def get_supported_configs(self) -> Optional[ProviderConfig]:
             ],
         )
 
+    def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
+        if VariantMeta(self.namespace, "key3", "val3a") in variant_desc:
+            return ["sec"]
+        return []
+
 
 class MockedPluginC(PluginBase):
-    namespace = "incompatible_plugin"
+    namespace = "other_plugin"
+
+    def get_supported_configs(self) -> Optional[ProviderConfig]:
+        return None
+
+    def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
+        ret = []
+        for meta in variant_desc:
+            if meta.namespace == self.namespace and meta.value == "on":
+                ret.append(meta.key)
+        return ret
+
+
+class MockedPluginD:
+    namespace = "plugin_without_labels"
 
     def get_supported_configs(self) -> Optional[ProviderConfig]:
         return None
@@ -48,6 +74,13 @@ class ClashingPlugin(PluginBase):
     def get_supported_configs(self) -> Optional[ProviderConfig]:
         return None
 
+    def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
+        ret = []
+        for meta in variant_desc:
+            if meta.namespace == self.namespace and meta.value == "on":
+                ret.append(meta.key)
+        return ret
+
 
 @dataclass
 class MockedDistribution:
@@ -87,6 +120,11 @@ def mocked_plugin_loader(session_mocker):
             value="tests.test_plugins:MockedPluginC",
             plugin=MockedPluginC,
         ),
+        MockedEntryPoint(
+            name="no_labels",
+            value="tests.test_plugins:MockedPluginD",
+            plugin=MockedPluginD,
+        ),
     ]
     yield PluginLoader()
 
@@ -137,3 +175,36 @@ def test_namespace_clash(mocker):
     assert "same namespace test_plugin" in str(exc)
     assert "test-plugin" in str(exc)
     assert "clashing-plugin" in str(exc)
+
+
+@pytest.mark.parametrize("variant_desc,expected",
+[
+    (VariantDescription([
+        VariantMeta("test_plugin", "key1", "val1a"),
+        VariantMeta("test_plugin", "key2", "val2b"),
+        VariantMeta("second_plugin", "key3", "val3a"),
+        VariantMeta("other_plugin", "flag2", "on"),
+    ]), ["1a", "sec", "flag2"]),
+    (VariantDescription([
+        # note that VariantMetas don't actually have to be supported
+        # by the system in question -- we could be cross-building
+        # for another system
+        VariantMeta("test_plugin", "key1", "val1f"),
+        VariantMeta("test_plugin", "key2", "val2b"),
+        VariantMeta("second_plugin", "key3", "val3a"),
+    ]), ["1f", "sec"]),
+    (VariantDescription([
+        VariantMeta("test_plugin", "key2", "val2b"),
+        VariantMeta("second_plugin", "key3", "val3a"),
+    ]), ["sec"]),
+    (VariantDescription([
+        VariantMeta("test_plugin", "key2", "val2b"),
+    ]), []),
+    (VariantDescription([
+        VariantMeta("test_plugin", "key2", "val2b"),
+        VariantMeta("other_plugin", "flag1", "on"),
+        VariantMeta("other_plugin", "flag2", "on"),
+    ]), ["flag1", "flag2"]),
+])
+def test_get_variant_labels(mocked_plugin_loader, variant_desc, expected):
+    assert mocked_plugin_loader.get_variant_labels(variant_desc) == expected

variantlib/base.py

@@ -2,9 +2,9 @@
 from typing import Protocol, runtime_checkable
 
 from variantlib.config import ProviderConfig
+from variantlib.meta import VariantDescription
 
 
-@runtime_checkable
 class PluginType(Protocol):
     """A protocol for plugin classes"""
 
@@ -17,6 +17,10 @@ def get_supported_configs(self) -> ProviderConfig:
         """Get supported configs for the current system"""
         ...
 
+    def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
+        """Get list of short labels to describe the variant"""
+        ...
+
 
 class PluginBase(ABC):
     """An abstract base class that can be used to implement plugins"""
@@ -26,3 +30,6 @@ def namespace(self) -> str: ...
 
     @abstractmethod
     def get_supported_configs(self) -> ProviderConfig: ...
+
+    def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
+        return []

variantlib/plugins.py

@@ -44,7 +44,12 @@ def load_plugins(self) -> None:
 
                 # Instantiate the plugin
                 plugin_instance = plugin_class()
-                assert isinstance(plugin_instance, PluginType)
+
+                # Check for obligatory members
+                for attr in ("namespace", "get_supported_configs"):
+                    assert hasattr(
+                        plugin_instance, attr
+                    ), f"Plugin is missing required member: {attr}"
             except Exception:
                 logging.exception("An unknown error happened - Ignoring plugin")
             else:
@@ -92,3 +97,13 @@ def get_dist_name_mapping(self) -> dict[str, str]:
         """Get a mapping from plugin names to distribution names"""
 
         return self._dist_names
+
+    def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
+        """Get list of short labels to describe the variant"""
+
+        labels = []
+        for plugin in self._plugins.values():
+            if hasattr(plugin, "get_variant_labels"):
+                labels += plugin.get_variant_labels(variant_desc)
+
+        return labels