Fix DunderMixin hash equality contract for dictionary merging purposes

steer_core/Mixins/Dunder.py

@@ -252,15 +252,17 @@ def __eq__(self, other):
         return True
 
     def __hash__(self):
-        """Hash based on object identity.
+        """Return a stable hash compatible with value-based equality.
 
-        Because the objects using this mixin are mutable, a content-based
-        hash would be unstable.  Using ``id()`` means that two distinct
-        objects that compare equal via ``__eq__`` will have different
-        hashes -- avoid using these objects as ``set`` members or ``dict``
-        keys when value-based identity matters.
+        Instances using this mixin are mutable, so hashing their property
+        values would make their hash unstable.  A constant hash preserves
+        the requirement that objects which compare equal have equal hashes
+        while allowing ``dict`` and ``set`` lookups to use ``__eq__``.
+
+        The tradeoff is that hash-based lookups among these objects are
+        linear rather than constant-time.
         """
-        return hash(id(self))
+        return 0
 
     def __str__(self):
         """
@@ -276,4 +278,3 @@ def __repr__(self):
         Official string representation of the instance.
         """
         return self.__str__()
-

test/test_dunder_mixin.py

@@ -33,12 +33,17 @@ def test_different_name(self, sample_obj):
 
 class TestDunderHash:
 
-    def test_hash_is_identity_based(self, sample_obj):
-        assert hash(sample_obj) == hash(id(sample_obj))
+    def test_equal_objects_have_equal_hashes(self, sample_obj):
+        equivalent = SampleObject(name="test", value=1.0)
 
-    def test_different_objects_different_hash(self, sample_obj):
-        other = SampleObject()
-        assert hash(sample_obj) != hash(other)
+        assert sample_obj == equivalent
+        assert hash(sample_obj) == hash(equivalent)
+
+    def test_equivalent_object_resolves_dict_key(self, sample_obj):
+        equivalent = SampleObject(name="test", value=1.0)
+        values = {sample_obj: 7}
+
+        assert values[equivalent] == 7
 
 
 class TestDunderStr: