pre_hooks are broken

[jdavies-st]

Step pre_hooks are broken currently, and we don't currently unit test them. post_hooks work fine. They were unit tested.

The recent removal of jwst as a test dependency masks the actual failure. So if we go back a few commits:

$ git reset 4667ada --hard

And then rewrite one of the unit tests:

$ git diff
diff --git a/tests/test_hooks.py b/tests/test_hooks.py
index 4d7d40a..6ff37d1 100644
--- a/tests/test_hooks.py
+++ b/tests/test_hooks.py
@@ -66,7 +66,7 @@ def test_hook_as_step_class(caplog):
 
     steps = {
         "cancelnoise": {
-            "post_hooks": [
+            "pre_hooks": [
                 HookStep,
             ]
         }

We get the following failure when running the unit tests:

$ pytest tests/test_hooks.py
...
FAILED tests/test_hooks.py::test_hook_as_step_class - TypeError: CancelNoiseStep.process() takes 2 positional arguments but 6 were given

This happens with real data and real steps as well if you try to use pre_hooks. Everything works fine with post_hooks. Not sure why the asymmetry.

h/t @TheSkyentist

[braingram]

Thanks for opening the issue.

Do you know if these ever worked? Looking at the code it seems like the expectation is that a pre-hook will return a tuple that replaces args:

stpipe/src/stpipe/step.py

Lines 463 to 468 in 799be65

	hook_args = args
	for pre_hook in self._pre_hooks:
	hook_results = pre_hook.run(*hook_args)
	if hook_results is not None:
	hook_args = hook_results
	args = hook_args

Which would cause issues using a Step as a pre-hook. If I try to change some of the tests in test_hooks (which no longer require jwst to run following #199) this leads to expected failures when Step instances used as pre-hooks don't return multiple values.