Add helper module for testing operator charms.

In our testing we often work around the fact that the operator
harness does not provide a `network_get()` by customizing the
_TestingModelBackend class which is a bit fragile. This helper module
implements a patch decorator for `network_get()` instead.

Change-Id: Iafabf81ecf3ea719c37144a24915a92643b98d7f

ops_openstack/testing.py

@@ -0,0 +1,32 @@
+# Copyright 2022 Canonical Ltd.
+# See LICENSE file for licensing details.
+"""Helpers for testing operator charms."""
+
+import typing
+from unittest import mock
+
+
+def patch_network_get(network_data=None) -> typing.Callable:
+    if network_data is None:
+        network_data = {
+            "bind-addresses": [
+                {
+                    "addresses": [{"value": "10.0.0.10"}],
+                }
+            ],
+        }
+
+    def network_get(*args, **kwargs) -> dict:
+        """Patch network_get.
+
+        Creates a patch for the not-yet-implemented testing backend needed
+        for `bind_address`.
+
+        This patch decorator can be used for cases such as:
+        self.model.get_binding(event.relation).network.bind_address
+        """
+        return network_data
+
+    return mock.patch(
+        "ops.testing._TestingModelBackend.network_get", network_get
+    )

unit_tests/test_ops_openstack.py

@@ -27,6 +27,7 @@ from ops.model import (
 )
 
 import ops_openstack.core
+from ops_openstack import testing
 
 
 class OpenStackTestPlugin1(ops_openstack.core.OSBaseCharm):
@@ -368,3 +369,13 @@ class TestOSBaseCharm(CharmTestCase):
         self.harness.update_config({'source': 'value'})
         self.assertTrue(
             isinstance(self.harness.charm.unit.status, ActiveStatus))
+
+
+class TestTesting(unittest.TestCase):
+
+    def test_patch_network_get(self):
+        network_data = {
+            'bind-addresses': [{'addresses': [{'value': '1.1.1.1'}]}],
+        }
+        with testing.patch_network_get(network_data) as p:
+            self.assertDictEqual(p(), network_data)