From 038ca6d5d72d6b42919686459f9d0d0986ad63e2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?=
 <marmarek@invisiblethingslab.com>
Date: Fri, 20 Oct 2017 01:24:43 +0200
Subject: [PATCH] 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   | 25 ++++++++++++++++++-------
 qubes/tests/init.py | 10 ++++++++++
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/qubes/__init__.py b/qubes/__init__.py
index a404f9a4..9d077c88 100644
--- a/qubes/__init__.py
+++ b/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:
-                raise AttributeError(
-                    'property {!r} not set'.format(self.__name__))
-            elif isinstance(self._default, collections.Callable):
-                return self._default(instance)
-            else:
-                return self._default
+            return self.get_default(instance)
 
+    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):
diff --git a/qubes/tests/init.py b/qubes/tests/init.py
index 25490c69..a2b9da3e 100644
--- a/qubes/tests/init.py
+++ b/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