I tried something like:

def test_single_file_not_found_gens_exception(monkeypatch):
     with pytest.raises(signals.FAIL):                                                                   
         flow.run(input_dir=os.getcwd() + "/test/input_files/",
                 file_name="does_not_exist")

I can run the test, and see:

prefect.TaskRunner | FAIL signal raised: FAIL('Expecting file: does_not_exist, not found in input_dir')

But, the test fails:

DID NOT RAISE <class 'prefect.engine.signals.FAIL'>

So, I thought maybe I should test the Task rather than run the flow.

I guess I can just ask the final state if it's failed.

def test_single_file_not_found_gens_exception(mock_nfs_mount):

state = flow.run(input_dir=os.getcwd() + "/test/input_files/",

file_name="does_not_exist")

assert state.is_failed()

It would nice if there was a clear example how you can test run a task, or a flow, to generate an exception.