Resource mocking for solid testing

[vilhub]

Hi,

Thanks for the great work on dagster, I have a question concerning how to best mock resources for testing solids individually. For now the approach I came up with is slightly too convoluted for my taste. Would you have a simpler approach or would this be a feature request to support resource mocking for testing solids?

Note that the resource should be configurable for every test. Here is the approach I have so far :

# Fixture which can be reused to mock any resource
@pytest.fixture(scope='function')
def mock_mode_definition(request):
    resource_name = request.param[0]
    service_mock_class = request.param[1]

    resource = dagster.resource(lambda _: service_mock_class())
    return ModeDefinition('test', resource_defs={
        resource_name: resource
    })

# Define this for each test
class MockService:
    def compute(self, *_, **__):
        return 'mock value'

@pytest.mark.parametrize('mock_mode_definition',
                         [('mock_resource_name', MockService)],
                         indirect=True)
def test_solid(mock_mode_definition):
    # Set up test
    res = execute_solid(solid_to_test,
                        input_values={'inputs': inputs},
                        mode_def=mock_mode_definition)
    # asserts

Thanks for your help!

[schrockn]

Hey @vilhub this is pretty much the best approach given the APIs we have today.

This is what I would do (without the pytest parameterization, which I think is the source of some of the convolution here):

from dagster import ResourceDefinition, ModeDefinition, solid, execute_solid


class MockService:
    def compute(self, *_, **__):
        return "mock value"


@solid(required_resource_keys={"service"})
def solid_to_test(context):
    return context.resources.service.compute()


def make_mock_def(**resource_instances):
    return ModeDefinition(
        resource_defs={
            key: ResourceDefinition.hardcoded_resource(resource_instance)
            for key, resource_instance in resource_instances.items()
        }
    )


def test_mock():
    result = execute_solid(solid_to_test, make_mock_def(service=MockService()))
    assert result.success
    assert result.output_value() == "mock value"

[vilhub]

Great, thanks a lot for the suggestion, this should work well! 👍

[mgasner]

With direct invocation, this is now even easier:

my_solid(resources={"key": resource_definition_or_resource})

[vilhub]

Thanks a lot for the update! Much easier indeed 👍