Python: Mocking an instance of a class inside a method using @patch

Question

I have a method foo in Python that creates a Service class. I want to mock the Service class but when I run the test, it still attempts to instantiate the class. Here is the simplified version of my setup:

class Service:
    def __init__(self, service):
        self.service_stuff = service

    def run_service(self):
        do_service_stuff()

def foo:
    new_service = Service("bar")
    new_service.run_service()

Then my unit test:

@patch('mymodule.service_file.Service')
def test_foo(self, mock_service):
    foo()

I would like to run foo, but have it use my mocked object instead of creating an actual Service instance, but instead, when I run it, it tries to instantiate an actual instance of Service() and runs foo() as usual even though it seems to recognize the string signature I've put into patch. Why is this happening?

nao · Accepted Answer · 2020-01-29 00:02:37Z

0

Figured it out: The patch reference to the class had to be of its imported name in the method itself rather than the original class, similar to https://stackoverflow.com/a/32461515/4234853

So the patch should look like: @patch('mymodule.foo_file.Service') instead of trying to patch the class directly.

answered Jan 29, 2020 at 0:02

nao

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chepner · Accepted Answer · 2020-01-29 00:06:30Z

0

In this case, it might be easier to make your function more test-friendly:

def foo(cls=Service):
    new_service = cls("bar")
    new_service.run_service()

Then your test doesn't need to patch anything.

def test_foo(self):
    mock_service = Mock()
    foo(mock_service)

answered Jan 29, 2020 at 0:06

chepner

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