The ImmutableDeclaration also has immutable _v_attrs.

Fixes #204

CHANGES.rst

@@ -45,6 +45,12 @@
 
   See `issue 3 <https://github.com/zopefoundation/zope.interface/issues/3>`_.
 
+- Make the internal singleton object returned by APIs like
+  ``implementedBy`` and ``directlyProvidedBy`` for objects that
+  implement or provide no interfaces more immutable. Previously an
+  internal cache could be mutated. See `issue 204
+  <https://github.com/zopefoundation/zope.interface/issues/204>`_.
+
 5.0.2 (2020-03-30)
 ==================
 

src/zope/interface/declarations.py

@@ -195,6 +195,18 @@ def weakref(self, callback=None):
         # object, and that includes a method.)
         return _ImmutableDeclaration
 
+    @property
+    def _v_attrs(self):
+        # _v_attrs is not a public, documented property, but some client
+        # code uses it anyway as a convenient place to cache things. To keep
+        # the empty declaration truly immutable, we must ignore that. That includes
+        # ignoring assignments as well.
+        return {}
+
+    @_v_attrs.setter
+    def _v_attrs(self, new_attrs):
+        pass
+
 
 ##############################################################################
 #

src/zope/interface/tests/test_declarations.py

@@ -122,6 +122,20 @@ def test___iro___(self):
         decl = self._getEmpty()
         self.assertEqual(decl.__iro__, (Interface,))
 
+    def test_get(self):
+        decl = self._getEmpty()
+        self.assertIsNone(decl.get('attr'))
+        self.assertEqual(decl.get('abc', 'def'), 'def')
+        # It's a positive cache only (when it even exists)
+        # so this added nothing.
+        self.assertFalse(decl._v_attrs)
+
+    def test_changed_w_existing__v_attrs(self):
+        decl = self._getEmpty()
+        decl._v_attrs = object()
+        decl.changed(decl)
+        self.assertFalse(decl._v_attrs)
+
 
 class DeclarationTests(EmptyDeclarationTests):
 
@@ -153,12 +167,6 @@ def test_changed_wo_existing__v_attrs(self):
         decl.changed(decl) # doesn't raise
         self.assertIsNone(decl._v_attrs)
 
-    def test_changed_w_existing__v_attrs(self):
-        decl = self._makeOne()
-        decl._v_attrs = object()
-        decl.changed(decl)
-        self.assertIsNone(decl._v_attrs)
-
     def test___contains__w_self(self):
         decl = self._makeOne()
         self.assertNotIn(decl, decl)
@@ -335,6 +343,19 @@ def test_get_always_default(self):
         self.assertIsNone(self._getEmpty().get('name'))
         self.assertEqual(self._getEmpty().get('name', 42), 42)
 
+    def test_v_attrs(self):
+        decl = self._getEmpty()
+        self.assertEqual(decl._v_attrs, {})
+
+        decl._v_attrs['attr'] = 42
+        self.assertEqual(decl._v_attrs, {})
+        self.assertIsNone(decl.get('attr'))
+
+        attrs = decl._v_attrs = {}
+        attrs['attr'] = 42
+        self.assertEqual(decl._v_attrs, {})
+        self.assertIsNone(decl.get('attr'))
+
 
 class TestImplements(NameAndModuleComparisonTestsMixin,
                      unittest.TestCase):