Raise when attaching partition keys to asset alias events

task-sdk/src/airflow/sdk/execution_time/context.py

@@ -977,7 +977,14 @@ def add_partitions(self, keys: str | list[str]) -> None:
 
         :raises ValueError: If any key is empty/whitespace-only or longer than
             ``_PARTITION_KEY_MAX_LENGTH`` characters.
+        :raises TypeError: If this accessor is for an asset alias, since partition
+            keys are only attached to concrete asset events, not alias events.
         """
+        if isinstance(self.key, AssetAliasUniqueKey):
+            raise TypeError(
+                "add_partitions() is not supported on asset alias outlet events; "
+                "partition keys can only be attached to a concrete asset."
+            )
         if isinstance(keys, str):
             keys = [keys]
         for key in keys:

task-sdk/tests/task_sdk/execution_time/test_context.py

@@ -536,6 +536,14 @@ def test_add_partitions_rejects_any_invalid_in_list(self, accessor):
             accessor.add_partitions(["us", ""])
         assert accessor.partition_keys == set()
 
+    def test_add_partitions_rejects_asset_alias_accessor(self):
+        alias_accessor = OutletEventAccessor(
+            key=AssetAliasUniqueKey.from_asset_alias(AssetAlias("test_alias"))
+        )
+        with pytest.raises(TypeError, match="not supported on asset alias"):
+            alias_accessor.add_partitions("us")
+        assert alias_accessor.partition_keys == set()
+
 
 class TestTriggeringAssetEventsAccessor:
     @pytest.fixture(autouse=True)