Here is my somewhat hack solution on how to 1. mock an operation when testing a job call 2. mock the operation context when testing a job call

from unittest import mock
from unittest.mock import patch

from dagster import (
    op,
    job,
)
from dagster_slack import slack_resource
from dagster._core.execution.plan.compute_generator import invoke_compute_fn


@op(required_resource_keys={"slack"})
def my_op(context):
    print(f'Inside the real operation: {context.op.name}')
    context.resources.slack.chat_postMessage(channel="#test42", text="test")


@op(required_resource_keys={"slack"})
def my_second_op(context):
    print(f'Inside the real second operation: {context.op.name}')


@job(resource_defs={"slack": slack_resource.configured({"token": "xoxb-123456"})})
def my_job():
    my_op()
    my_second_op()


def test_my_job():
    """Mock the context of the operation my_op within a job and completely mock the operation my_second_op."""
    mocked_context = mock.MagicMock()
    my_second_op_mock = mock.MagicMock()

    def invoke_op_with_mocked_context(fn, context, kwargs, context_arg_provided, config_arg_cls, resource_args=None):
        
        # Mock specific operations (or mock all of them by replacing fn).
        if context.op.name == 'my_second_op':
            fn = my_second_op_mock

        # Use this to mock the context for all operations
        context = mocked_context

        return invoke_compute_fn(fn, context, kwargs, context_arg_provided, config_arg_cls, resource_args)

    with patch('dagster._core.execution.plan.compute_generator.invoke_compute_fn') as mocked_invoke_compute_fn:
        mocked_invoke_compute_fn.side_effect = invoke_op_with_mocked_context
        my_job.execute_in_process()

        assert mocked_context.resources.slack.chat_postMessage.called
        assert my_second_op_mock.called