Python patch object, how to assert on calls

Question

I'm trying to create some pytest unit tests on a custom ansible module. I'm trying to mock the boto3 client and just test the logic of the class. I moved the initialization of boto into a class method so I can patch the class and replace it with a mock object.

Patch seems to be successful, the "_init_boto" method is returning a mock object. But I can't figure out how to set the return value so that "describe_stacks(..)" returns a custom reponse, or assert on the calls on the mock. Sample code:

cf_stack_exists.py:

import boto3
class CfStackExistsSimplified:

    def __init__(self, aws_profile) -> None:
        super().__init__()
        self.aws_profile = aws_profile
        self.boto_client = self._init_boto(aws_profile)

    def _init_boto(self):
        session = boto3.session.Session(profile_name=self.aws_profile)
        return session.client('cloudformation')

    def exec(self):
        response = self.boto_client.describe_stacks(
            StackName=self.aws_profile
        )
        return response

test_myclass.py:

from unittest.mock import patch
from cf_stack_exists import CfStackExistsSimplified

class TestCfStackExists:
    @patch.object(CfStackExistsSimplified, "_init_boto")
    def test_example(self, mock_init_boto):
        myclass = CfStackExistsSimplified(aws_profile="dev")
        response = myclass.exec()
        print(f"\n{mock_init_boto.mock_calls}\n")
        mock_init_boto.describe_stacks().assert_called()

When I run this it fails with the error:

...
>           raise AssertionError(msg)
E           AssertionError: Expected 'mock' to have been called.
...

But the print statement is showing that it has been called?

[call('dev'), call().describe_stacks(StackName='dev')]

I really can't figure out how to assert this mock, tried lots of different assert commands. Any help much appreciated!!

blhsing · Accepted Answer · 2023-09-20 03:48:15Z

1

By placing parentheses after the mocked function object describe_stacks, you're calling the function again, getting the child Mock object returned by the mock call, not the mocked function object that has the call records.

To assert that a mocked function is called, call the assert_called method of the mocked function object itself.

Conversely, to access a child Mock object returned by a call to a Mock object, which is the case of self.boto_client since it is assigned with what's returned by calling self._init_boto (mocked as mock_init_boto), you should place parentheses after the parent Mock object.

Change:

mock_init_boto.describe_stacks().assert_called()

to:

mock_init_boto().describe_stacks.assert_called()

edited Sep 20, 2023 at 3:48

answered Sep 20, 2023 at 2:39

blhsing

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