Don't propagate fallbacks to subclasses.

README.rst

@@ -539,7 +539,7 @@ Methods and properties defined on the delegate class itself take precedence (as
             return 'my bar'
 
 However, attempting to set an instance attribute as an override will just set the attribute on the underlying delegate
-instead.  If you want to override using an instance attribute, first define it as a class attribute::
+instead.  If you want to override an interface attribute using an instance attribute, first define it as a class attribute::
 
     class MyDelegate(Delegate, IFoo):
         pi_attr_delegates = {'impl': IFoo}

pure_interface/__init__.py

@@ -11,4 +11,4 @@
 except ImportError:
     pass
 
-__version__ = '7.0.2'
+__version__ = '7.1.0'

pure_interface/delegation.py

@@ -2,6 +2,7 @@
 
 import operator
 
+import pure_interface
 from .errors import InterfaceError
 from .interface import get_interface_names, type_is_interface, get_type_interfaces, InterfaceType
 
@@ -123,7 +124,7 @@ def i_have_attribute(attrib):
                     return True
             return False
 
-        for delegate, attr_list in cls.pi_attr_delegates.items():
+        for delegate, attr_list in cls.__dict__.get('pi_attr_delegates', {}).items():
             if isinstance(attr_list, type):
                 attr_list = list(get_interface_names(attr_list))
             if delegate in cls.pi_attr_mapping:
@@ -135,11 +136,11 @@ def i_have_attribute(attrib):
                     continue
                 dotted_name = f'{delegate}.{attr}'
                 setattr(cls, attr, _Delegated(dotted_name))
-        for attr, dotted_name in cls.pi_attr_mapping.items():
+        for attr, dotted_name in cls.__dict__.get('pi_attr_mapping', {}).items():
             if not i_have_attribute(attr):
                 setattr(cls, attr, _Delegated(dotted_name))
-        if cls.pi_attr_fallback:
-            fallback = cls.pi_attr_fallback
+        fallback = cls.__dict__.get('pi_attr_fallback', None)
+        if fallback is not None:
             for interface in get_type_interfaces(cls):
                 interface_names = get_interface_names(interface)
                 for attr in interface_names:

setup.cfg

@@ -1,6 +1,6 @@
 [metadata]
 name = pure_interface
-version = 7.0.2
+version = 7.1.0
 description = A Python interface library that disallows function body content on interfaces and supports adaption.
 keywords = abc interface adapt adaption mapper structural typing dataclass
 author = Tim Mitchell

tests/test_delegate.py

@@ -73,6 +73,11 @@ class DSubFallback(DFallback, ISubTalker):
     pass
 
 
+class DSubFallback2(DFallback, ISubTalker):
+    pi_attr_fallback = 'impl'
+
+
+
 class DAttrMap(delegation.Delegate, IPoint):
     pi_attr_mapping = {'x': 'a.x',
                        'y': 'b.y',
@@ -287,8 +292,13 @@ def test_delegate_subclass(self):
         self.assertEqual(3, d3.z)
 
     def test_delegate_subclass_fallback(self):
-        """Fallback delegates are used for subclass interface attributes too."""
-        d = DSubFallback(Talker())
+        """Check fallback delegates are not used for subclass interface attributes too."""
+        with self.assertRaises(TypeError):
+            DSubFallback(Talker())  # no implementation of chat
+
+    def test_delegate_subclass_fallback2(self):
+        """Check subclass fallbacks are used for missing attributes."""
+        d = DSubFallback2(Talker())
         self.assertEqual('chat', d.chat())