How to use the ops.testing.Harness function in ops

def test_update_relation_no_local_app_change_event(self):
        # language=YAML
        harness = Harness(CharmBase, meta='''
            name: my-charm
            requires:
              db:
                interface: pgsql
            ''')
        self.addCleanup(harness.cleanup)
        harness.begin()
        harness.set_leader(False)
        helper = DBRelationChangedHelper(harness.charm, "helper")
        rel_id = harness.add_relation('db', 'postgresql')
        # TODO: remove this as soon as https://github.com/canonical/operator/issues/175 is fixed.
        harness.add_relation_unit(rel_id, 'postgresql/0')
        self.assertEqual(helper.changes, [])

        harness.update_relation_data(rel_id, 'my-charm', {'new': 'value'})
        rel = harness.charm.model.get_relation('db')

def test_relation_set_deletes(self):
        harness = Harness(CharmBase, meta='''
            name: test-charm
            requires:
                db:
                    interface: pgsql
            ''')
        self.addCleanup(harness.cleanup)
        harness.begin()
        harness.set_leader(False)
        rel_id = harness.add_relation('db', 'postgresql')
        harness.update_relation_data(rel_id, 'test-charm/0', {'foo': 'bar'})
        harness.add_relation_unit(rel_id, 'postgresql/0')
        rel = harness.charm.model.get_relation('db', rel_id)
        del rel.data[harness.charm.model.unit]['foo']
        self.assertEqual({}, harness.get_relation_data(rel_id, 'test-charm/0'))

def test_add_relation_and_unit(self):
        harness = Harness(CharmBase, meta='''
            name: test-app
            requires:
                db:
                    interface: pgsql
            ''')
        self.addCleanup(harness.cleanup)
        rel_id = harness.add_relation('db', 'postgresql')
        self.assertIsInstance(rel_id, int)
        harness.add_relation_unit(rel_id, 'postgresql/0')
        harness.update_relation_data(rel_id, 'postgresql/0', {'foo': 'bar'})
        backend = harness._backend
        self.assertEqual(backend.relation_ids('db'), [rel_id])
        self.assertEqual(backend.relation_list(rel_id), ['postgresql/0'])
        self.assertEqual(
            backend.relation_get(rel_id, 'postgresql/0', is_app=False),
            {'foo': 'bar'})

def test_status_set_get_app(self):
        harness = Harness(CharmBase, meta='''
            name: app
            ''')
        self.addCleanup(harness.cleanup)
        backend = harness._backend
        backend.status_set('blocked', 'message', is_app=True)
        self.assertEqual(
            backend.status_get(is_app=True),
            {'status': 'blocked', 'message': 'message'})
        self.assertEqual(
            backend.status_get(is_app=False),
            {'status': 'maintenance', 'message': ''})

def test_add_resource_but_oci(self):
        harness = Harness(CharmBase, meta='''
            name: test-app
            resources:
              image:
                type: oci-image
                description: "Image to deploy."
            ''')
        self.addCleanup(harness.cleanup)
        with self.assertRaises(RuntimeError):
            harness.add_resource('image', 'content')

def test_relation_list_unknown_relation_id(self):
        harness = Harness(CharmBase, meta='''
            name: test-charm
            ''')
        self.addCleanup(harness.cleanup)
        backend = harness._backend
        with self.assertRaises(RelationNotFoundError):
            backend.relation_list(1234)

def test_add_resource_unknown(self):
        harness = Harness(CharmBase, meta='''
            name: test-app
            resources:
              image:
                type: file
                description: "Image to deploy."
            ''')
        self.addCleanup(harness.cleanup)
        with self.assertRaises(RuntimeError):
            harness.add_resource('unknown', 'content')

def test_set_model_name_after_begin(self):
        harness = Harness(CharmBase, meta='''
            name: test-charm
        ''')
        self.addCleanup(harness.cleanup)
        harness.set_model_name('bar')
        harness.begin()
        with self.assertRaises(RuntimeError):
            harness.set_model_name('foo')
        self.assertEqual(harness.model.name, 'bar')

def test_add_resource_unknown_filename(self):
        harness = Harness(CharmBase, meta='''
            name: test-app
            resources:
              image:
                type: file
                description: "Image to deploy."
            ''')
        self.addCleanup(harness.cleanup)
        harness.add_resource('image', 'foo contents\n')
        path = harness.model.resources.fetch('image')
        self.assertTrue(
            str(path).endswith('/image/image'),
            msg='expected {} to end with /image/image')

def test_add_resource_string(self):
        harness = Harness(CharmBase, meta='''
            name: test-app
            resources:
              image:
                type: file
                filename: foo.txt
                description: "Image to deploy."
            ''')
        self.addCleanup(harness.cleanup)
        harness.add_resource('image', 'foo contents\n')
        path = harness.model.resources.fetch('image')
        self.assertTrue(
            str(path).endswith('/image/foo.txt'),
            msg='expected {} to end with /image/foo.zip')
        with path.open('rt') as f:
            self.assertEqual('foo contents\n', f.read())