base: add method for getting default property value

Allow to get default value even it isn't set currently. This will allow
(G)UI to present better view, without duplicating logic for default
value.

Fixes QubesOS/qubes-issues#3197

qubes/__init__.py

@@ -221,14 +221,16 @@ class property(object):  # pylint: disable=redefined-builtin,invalid-name
             return getattr(instance, self._attr_name)
 
         except AttributeError:
-            if self._default is self._NO_DEFAULT:
+            return self.get_default(instance)
-                raise AttributeError(
-                    'property {!r} not set'.format(self.__name__))
-            elif isinstance(self._default, collections.Callable):
-                return self._default(instance)
-            else:
-                return self._default
 
+    def get_default(self, instance):
+        if self._default is self._NO_DEFAULT:
+            raise AttributeError(
+                'property {!r} have no default'.format(self.__name__))
+        elif isinstance(self._default, collections.Callable):
+            return self._default(instance)
+        else:
+            return self._default
 
     def __set__(self, instance, value):
         self._enforce_write_once(instance)
@@ -554,6 +556,15 @@ class PropertyHolder(qubes.events.Emitter):
         attrname = self.property_get_def(prop)._attr_name
         return not hasattr(self, attrname)
 
+    def property_get_default(self, prop):
+        '''Get property default value.
+
+        :param qubes.property or str prop: property object of particular
+        interest
+        '''
+
+        return self.property_get_def(prop).get_default(self)
+
 
     @classmethod
     def property_get_def(cls, prop):

qubes/tests/init.py

@@ -100,6 +100,9 @@ class TC_10_property(qubes.tests.QubesTestCase):
         holder = MyTestHolder(None)
 
         self.assertEqual(holder.testprop1, 'defaultvalue')
+        self.assertEqual(
+            type(holder).testprop1.get_default(holder),
+            'defaultvalue')
 
     def test_023_get_default_func(self):
         class MyTestHolder(qubes.tests.TestEmitter, qubes.PropertyHolder):
@@ -278,6 +281,13 @@ class TC_20_PropertyHolder(qubes.tests.QubesTestCase):
         expected_prop3.text = 'testdefault'
         self.assertXMLEqual(elements_with_defaults[2], expected_prop3)
 
+    def test_007_property_get_default(self):
+        self.assertEqual(
+            self.holder.property_get_default('testprop3'),
+            'testdefault')
+        with self.assertRaises(AttributeError):
+            self.holder.property_get_default('testprop1'),
+
     @unittest.skip('test not implemented')
     def test_010_property_require(self):
         pass