diff --git a/staticconf/testing.py b/staticconf/testing.py
index 4a3bcc6..e85a594 100644
--- a/staticconf/testing.py
+++ b/staticconf/testing.py
@@ -1,11 +1,13 @@
 """
 Facilitate testing of code which uses staticconf.
 """
+import copy
+
 from staticconf import config, loader
 
 
 class MockConfiguration(object):
-    """A a context manager which patches the configuration namespace
+    """A context manager which replaces the configuration namespace
     while inside the context. When the context exits the old configuration
     values will be restored to that namespace.
 
@@ -56,3 +58,35 @@ def __enter__(self):
 
     def __exit__(self, *args):
         self.teardown()
+
+
+class PatchConfiguration(MockConfiguration):
+    """A context manager which updates the configuration namespace while inside
+    the context. When the context exits the old configuration values will be
+    restored to that namespace.
+
+    Unlike MockConfiguration which completely replaces the configuration with
+    the new one, this class instead only updates the keys in the configuration
+    which are passed to it.  It preserves all previous values that weren't
+    updated.
+
+    .. code-block:: python
+
+        import staticconf.testing
+
+        config = {
+            ...
+        }
+        with staticconf.testing.PatchConfiguration(config, namespace='special'):
+            # Run your tests.
+        ...
+
+    The arguments are identical to MockConfiguration.
+    """
+
+    def setup(self):
+        self.old_values = copy.deepcopy(dict(self.namespace.get_config_values()))
+        new_configuration = copy.deepcopy(self.old_values)
+        new_configuration.update(self.config_data)
+        self.reset_namespace(new_configuration)
+        config.reload(name=self.namespace.name)
diff --git a/tests/testing_test.py b/tests/testing_test.py
index be9fd41..8775c43 100644
--- a/tests/testing_test.py
+++ b/tests/testing_test.py
@@ -22,3 +22,20 @@ def test_init_nested(self):
         with testing.MockConfiguration(conf):
             assert_equal(staticconf.get('a.b'), 'two')
             assert_equal(staticconf.get('c'), 'three')
+
+
+class TestPatchConfiguration(object):
+
+    def test_nested(self):
+        with testing.MockConfiguration(a='one', b='two'):
+            with testing.PatchConfiguration(a='three'):
+                assert_equal(staticconf.get('a'), 'three')
+                assert_equal(staticconf.get('b'), 'two')
+
+            assert_equal(staticconf.get('a'), 'one')
+            assert_equal(staticconf.get('b'), 'two')
+
+    def test_not_nested(self):
+        with testing.PatchConfiguration(a='one', b='two'):
+            assert_equal(staticconf.get('a'), 'one')
+            assert_equal(staticconf.get('b'), 'two')