I start this post by saying I'm not a professional software developer, I work mainly in IT Operations, although I write especially for IAC and small lambdas functions.

When developing a Lambda function most of the time I need to interact with AWS Services via the famousboto3 library;boto3 is a powerful library developed and maintained by AWS which provides a communication framework to interact with native AWS Cloud Services.

Every time I struggle to mock the library. I even considered using themoto stubber but, I'm not happy with what is provided. I aim to

1. Create a mock which can stub the original call
2. I want the mock to expose all the methods to check which parameters have been used during the calls, how many times it has been called, etc... all the stuff included within theMock class

In this example, we will mockboto3 while it creates a client forRDS.

Consider the following code

importboto3classMyRDSManager:def__init__(self)->None:self._rds_client=boto3.client("rds")defdelete_db_cluster_snapshot(self)->None:self._rds_client.delete_db_cluster_snapshot(DBClusterSnapshotIdentifier="ABC")defget_snapshots(self)->list[dict]:snapshots=[]paginator=self._rds_client.get_paginator("describe_db_cluster_snapshots")pages=paginator.paginate(DBClusterIdentifier="MyCluster")forpageinpages:page_snapshots=page.get("DBClusterSnapshots")forsnapshotinpage_snapshots:snapshots.append(snapshot)returnsnapshots

To test the above class I developed the following tests

importpytestfromunittest.mockimportMock,patchfromdatetimeimportdatetimefrommainimportMyRDSManager@pytest.fixture(scope="function")defprepare_mock():withpatch("main.boto3.client")asmock_boto_client:# the first thing to do is to set the return_value attribute as itself# this will return the mock itself when the code runs `boto3.client("rds")`mock_boto_client.return_value=mock_boto_clientdefmock_paginate_describe_db_cluster_snapshots(*args,**kwargs):snapshot_type=kwargs.get("SnapshotType")# get all the parameter passed to the callreturn[{"DBClusterSnapshots":[{"DBClusterSnapshotIdentifier":"ABC","SnapshotCreationTime":"2024-01-01","SnapshotType":"manual",}]}]mock_boto_client.get_paginator=Mock()mock_paginator=Mock(return_value=None)mock_paginator.paginate=Mock(return_value=None)mock_paginator.paginate.side_effect=mock_paginate_describe_db_cluster_snapshotsmock_boto_client.get_paginator.return_value=mock_paginatormock_boto_client.delete_db_cluster_snapshot=Mock(return_value=None)my_rds=MyRDSManager()yieldmy_rds,mock_boto_clientdeftest_one(prepare_mock):mock_my_rds,mock_boto_client=prepare_mockmock_my_rds.delete_db_cluster_snapshot()mock_boto_client.delete_db_cluster_snapshot.assert_called_once_with(DBClusterSnapshotIdentifier="ABC")result=mock_my_rds.get_snapshots()assertresult==[{"DBClusterSnapshotIdentifier":"ABC","SnapshotCreationTime":"2024-01-01","SnapshotType":"manual",}]

The first thing to notice is that we need to set themock_boto_client.return_value tomock_boto_client, which is itself, and this will return the mockinstance you are configuring when inMyRDSManager the code runsself._rds_client = boto3.client("rds"). If you don't set this MagicMock will return the default value, therefore anew MagicMock, and not what you are configuring!

Another important point to note is that we patchboto3 within the specific module being tested, rather than thegeneralboto3 library. In other words, you should patchmain.boto3.client, wheremain refers to the module you have written and are currently testing.

Next, configure the Mock as needed by adding methods and attributes. It's worth noting that setting aside_effect allows the mock to invoke the function specified in theside_effect, passing along all the arguments that the code supplies to the mock.

With this setup, you should be able to fulfil the initial requirements and therefore stub the original behaviour and use all the Mock-provided features.

Hope this will help you all!

Any feedback is appreciated.

Cheers

Post published also on Virtuability's websitehere.