Allow field(on_setattr=NO_OP) on frozen classes

Author: hynek

changelog.d/1515.change.md

@@ -0,0 +1 @@
+Frozen classes can set `on_setattr=attrs.setters.NO_OP` in addition to `None`.

src/attr/_make.py

@@ -2022,7 +2022,7 @@ def _make_init_script(
         attr_dict[a.name] = a
 
         if a.on_setattr is not None:
-            if frozen is True:
+            if frozen is True and a.on_setattr is not setters.NO_OP:
                 msg = "Frozen classes can't use on_setattr."
                 raise ValueError(msg)
 

tests/test_setattr.py

@@ -221,6 +221,16 @@ class C:
 
         assert "Frozen classes can't use on_setattr." == ei.value.args[0]
 
+    @pytest.mark.parametrize("nop", [None, setters.NO_OP])
+    def test_frozen_on_setattr_nops(self, nop):
+        """
+        on_setattr on frozen classes can be used for None and NO_OP.
+        """
+
+        @attr.s(frozen=True)
+        class C:
+            x = attr.ib(on_setattr=nop)
+
     def test_setattr_reset_if_no_custom_setattr(self, slots):
         """
         If a class with an active setattr is subclassed and no new setattr