Add context() method to registry.Value objects. Closes GH-430.

This method returns a context objet, for use with the 'with' statement.

src/registry.py

@@ -360,6 +360,18 @@ class Value(Group):
         for callback, args, kwargs in self._callbacks:
             callback(*args, **kwargs)
 
+    def context(self, value):
+        """Return a context manager object, which sets this variable to a
+        temporary value, and set the previous value back when exiting the
+        context."""
+        class Context:
+            def __enter__(self2):
+                self2._old_value = self.value
+                self.setValue(value)
+            def __exit__(self2, exc_type, exc_value, traceback):
+                self.setValue(self2._old_value)
+        return Context()
+
     def addCallback(self, callback, *args, **kwargs):
         """Add a callback to the list. A callback is a function that will be
         called when the value is changed. You can give this function as many

test/test_registry.py

@@ -174,4 +174,11 @@ class ValuesTestCase(SupyTestCase):
         registry.open_registry(filename)
         self.assertEqual(conf.supybot.reply.whenAddressedBy.chars(), '\\')
 
+    def testWith(self):
+        v = registry.String('foo', 'help')
+        self.assertEqual(v(), 'foo')
+        with v.context('bar'):
+            self.assertEqual(v(), 'bar')
+        self.assertEqual(v(), 'foo')
+
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