Mock or patch a class instance variable in python?

Question

So I have a class S3 in file/s3.py with the following init:

class S3:
    def __init__(self):
        self.s3_client = boto3.client("s3")

    def get_object(self):
        s3_object = self.s3_client.get_object()
        return s3_object

As you can see, the boto3.client class has a method named get_object() as well. I'm testing my custom get_object() method from another file but don't want to actaully make a boto3.client call to AWS services.

test.py

class TestS3Class(unittest.TestCase):
    """TestCase for file/s3.py"""

    def setUp(self):
        """Creates an instance of the live S3 class for testing"""
        self.s3_test_client = S3()

    @patch('boto3.client')
    def test_get_object(self, mock_client):
        """"""
        mock_client.get_object.return_value = {'key': 'value'}
        self.assertIsInstance(self.s3_test_client.get_object(), dict)

At the moment I'm getting AssertionError: None is not an instance of <class 'dict'> as a response, so I'm not sure if I'm patching something or not, but it's definitely not setting boto3.client.get_object()'s return value to the dictionary object.

Any hints on how to mock that self.s3_client = boto3.client()?

I fear it has something to do with the fact that my class function is named the same as the boto3.client method, get_object(). Don't know what's wrong though — sam
– sam, Commented Oct 19, 2019 at 21:33

blhsing · Accepted Answer · 2019-10-19 22:13:40Z

1

boto3.client returns an instance of a dynamically-created class based on the first argument (see source), so you cannot use the patch method, which requires that the target object be importable.

Instead, you can monkey-patch the get_object attribute of the class object of the client with a Mock object that returns your desired dict when called:

class TestS3Class(unittest.TestCase):
    def setUp(self):
        self.s3_test_client = S3()
        self.s3_test_client.s3_client.__class__.get_object = Mock(return_value={'key': 'value'})

    def test_get_object(self):
        self.assertIsInstance(self.s3_test_client.get_object(), dict)

Demo: https://repl.it/@blhsing/MarriedDeficientNumerator

edited Oct 19, 2019 at 22:13

answered Oct 19, 2019 at 22:02

blhsing

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2 Comments

sam Over a year ago

Ah dang, I burned quite a bit of time trying to figure that out. Came across this hint about runtime a little while ago too, but it didn't register. Yep, that seems to have worked. Thanks @blhsing.

sam Over a year ago

Oh, and I guess I should have guessed from the fact that it takes an argument ec2, s3, etc to create the right client class...

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