adds Harness.hooks_disabled, a context manager (#393)

* adds Harness.hooks_disabled, a context manager

ops/testing.py

@@ -15,10 +15,11 @@
 import inspect
 import pathlib
 import random
-from textwrap import dedent
 import tempfile
 import typing
 import yaml
+from contextlib import contextmanager
+from textwrap import dedent
 
 from ops import (
     charm,
@@ -321,6 +322,24 @@ def enable_hooks(self) -> None:
         """
         self._hooks_enabled = True
 
+    @contextmanager
+    def hooks_disabled(self):
+        """A context manager to run code with hooks disabled.
+
+        Example::
+
+            with harness.hooks_disabled():
+                # things in here don't fire events
+                harness.set_leader(True)
+                harness.update_config(unset=['foo', 'bar'])
+            # things here will again fire events
+        """
+        self.disable_hooks()
+        try:
+            yield None
+        finally:
+            self.enable_hooks()
+
     def _next_relation_id(self):
         rel_id = self._relation_id_counter
         self._relation_id_counter += 1

test/test_testing.py

@@ -540,6 +540,28 @@ def test_hooks_enabled_and_disabled(self):
             harness.charm.get_changes(reset=True),
             [{'name': 'config-changed', 'data': {'value': 'fourth', 'third': '3'}}])
 
+    def test_hooks_disabled_contextmanager(self):
+        harness = Harness(RecordingCharm, meta='''
+            name: test-charm
+        ''')
+        self.addCleanup(harness.cleanup)
+        # Before begin() there are no events.
+        harness.update_config({'value': 'first'})
+        # By default, after begin the charm is set up to receive events.
+        harness.begin()
+        harness.update_config({'value': 'second'})
+        self.assertEqual(
+            harness.charm.get_changes(reset=True),
+            [{'name': 'config-changed', 'data': {'value': 'second'}}])
+        # Once disabled, we won't see config-changed when we make an update
+        with harness.hooks_disabled():
+            harness.update_config({'third': '3'})
+        self.assertEqual(harness.charm.get_changes(reset=True), [])
+        harness.update_config({'value': 'fourth'})
+        self.assertEqual(
+            harness.charm.get_changes(reset=True),
+            [{'name': 'config-changed', 'data': {'value': 'fourth', 'third': '3'}}])
+
     def test_metadata_from_directory(self):
         tmp = pathlib.Path(tempfile.mkdtemp())
         self.addCleanup(shutil.rmtree, str(tmp))