Add a convenience function to just patch a single thing.

testtools/monkey.py

@@ -2,6 +2,11 @@
 
 """Helpers for monkey-patching Python code."""
 
+__all__ = [
+    'MonkeyPatcher',
+    'patch',
+    ]
+
 
 class MonkeyPatcher(object):
     """A set of monkey-patches that can be applied and removed all together.
@@ -73,3 +78,20 @@ class MonkeyPatcher(object):
             return f(*args, **kw)
         finally:
             self.restore()
+
+
+def patch(obj, attribute, value):
+    """Set 'obj.attribute' to 'value' and return a callable to restore 'obj'.
+
+    If 'attribute' is not set on 'obj' already, then the returned callable
+    will delete the attribute when called.
+
+    :param obj: An object to monkey-patch.
+    :param attribute: The name of the attribute to patch.
+    :param value: The value to set 'obj.attribute' to.
+    :return: A nullary callable that, when run, will restore 'obj' to its
+        original state.
+    """
+    patcher = MonkeyPatcher((obj, attribute, value))
+    patcher.patch()
+    return patcher.restore

testtools/tests/test_monkey.py

@@ -4,7 +4,7 @@
 """Tests for testtools.monkey."""
 
 from testtools import TestCase
-from testtools.monkey import MonkeyPatcher
+from testtools.monkey import MonkeyPatcher, patch
 
 
 class TestObj:
@@ -143,6 +143,24 @@ class MonkeyPatcherTest(TestCase):
         self.assertEquals(self.test_object.bar, self.original_object.bar)
 
 
+class TestPatchHelper(TestCase):
+
+    def test_patch_patches(self):
+        # patch(obj, name, value) sets obj.name to value.
+        test_object = TestObj()
+        patch(test_object, 'foo', 42)
+        self.assertEqual(42, test_object.foo)
+
+    def test_patch_returns_cleanup(self):
+        # patch(obj, name, value) returns a nullary callable that restores obj
+        # to its original state when run.
+        test_object = TestObj()
+        original = test_object.foo
+        cleanup = patch(test_object, 'foo', 42)
+        cleanup()
+        self.assertEqual(original, test_object.foo)
+
+
 def test_suite():
     from unittest import TestLoader
     return TestLoader().loadTestsFromName(__name__)