Allow deletion of frozen fields

semimutable/__init__.py

@@ -94,6 +94,17 @@ def __set__(self, instance: object, value: T) -> None:
 
         setattr(instance, self._private_name, value)
 
+    def __delete__(self, instance: object) -> None:
+        try:
+            delattr(instance, self._private_name)
+        except AttributeError:
+            pass
+        public_name = self._private_name[len(FROZEN_PREFIX) :]
+        try:
+            instance.__dict__.pop(public_name, None)
+        except AttributeError:
+            pass
+
 
 error = RuntimeError(
     "This field is created via field(frozen=True) but the @semimutable.dataclass decorator is not used on the dataclass. "

tests/test_semimutable_dataclass.py

@@ -27,6 +27,17 @@ class Sm:
     sm.y = 42
 
 
+def test_frozen_field_can_be_deleted():
+    @dataclass(slots=True)
+    class Sm:
+        x: int = field(frozen=True)
+
+    sm = Sm(x=1)
+    del sm.x
+    with pytest.raises(AttributeError):
+        sm.x
+
+
 def test_plain_dataclass_is_refused():
     with pytest.raises(RuntimeError):