Increase robustness of sentinel equality checks.

spec_classes/types/missing.py

@@ -16,6 +16,12 @@ def __bool__(cls):
     def __call__(cls):
         return cls
 
+    def __hash__(cls):
+        return id(cls)
+
+    def __eq__(cls, other):
+        return cls is other
+
 
 class MISSING(metaclass=_MissingType):
     """

spec_classes/types/spec_property.py

@@ -6,6 +6,7 @@
 from cached_property import cached_property
 
 from spec_classes.errors import NestedAttributeError
+from spec_classes.types.missing import EMPTY, MISSING, UNCHANGED
 from spec_classes.utils.mutation import prepare_attr_value
 from spec_classes.utils.stackdepth import get_spec_classes_depth
 from spec_classes.utils.type_checking import check_type, type_label
@@ -276,7 +277,7 @@ def __get__(self, instance, owner=None):
                 )
 
         # Store value in cache is cache is enabled
-        if self.cache:
+        if self.cache and value not in (MISSING, EMPTY, UNCHANGED):
             instance.__dict__[self.attr_name] = value
 
         return value

spec_classes/utils/mutation.py

@@ -227,7 +227,7 @@ def mutate_value(
     # `replace`; otherwise use MISSING.
     if new_value not in (MISSING, EMPTY, UNCHANGED):
         value = new_value
-    elif new_value is UNCHANGED or not replace:
+    elif not replace:
         value = old_value
         prepare = None  # Old values have already been prepared, so we suppress further preparation.
     else: