feat: add a __str__ to ActionFailed, for better unexpected failure …

…output (#1209)

Add a `__str__` to `ops.testing.ActionFailed` so that an uncaught
`event.fail()` in tests produces a more informative traceback.

Currently, `event.fail()` and `event.fail(message)` will look somewhat
like:

```python
Traceback (most recent call last):
  File "/tmp/1204.py", line 18, in <module>
    harness.run_action("act")
  File "/home/tameyer/code/operator/ops/testing.py", line 2001, in run_action
    raise ActionFailed(
ops.testing.ActionFailed
```

After this PR, `event.fail()` will look like:

```python
Traceback (most recent call last):
  File "/tmp/1204.py", line 19, in <module>
    harness.run_action("act")
  File "/home/tameyer/code/operator/ops/testing.py", line 2006, in run_action
    raise ActionFailed(
ops.testing.ActionFailed: Event handler called `fail()` with no additional details.
```

and `event.fail("Message")` will look like:

```python
Traceback (most recent call last):
  File "/tmp/1204.py", line 18, in <module>
    harness.run_action("act")
  File "/home/tameyer/code/operator/ops/testing.py", line 2006, in run_action
    raise ActionFailed(
ops.testing.ActionFailed: Message

```

Fixes #1204

---------

Co-authored-by: Tiexin Guo <tiexin.guo@canonical.com>

ops/testing.py

@@ -166,6 +166,11 @@ def __init__(self, message: str, output: ActionOutput):
         self.message = message
         self.output = output
 
+    def __str__(self):
+        if self.message:
+            return self.message
+        return "Event handler called `fail()` with no additional details."
+
 
 @dataclasses.dataclass()
 class _RunningAction:

test/test_testing.py

@@ -5584,7 +5584,10 @@ def _on_simple_action(self, event: ops.ActionEvent):
             def _on_fail_action(self, event: ops.ActionEvent):
                 event.fail("this will be ignored")
                 event.log("some progress")
-                event.fail("something went wrong")
+                if event.params.get('empty-failure-message'):
+                    event.fail()
+                else:
+                    event.fail("something went wrong")
                 event.log("more progress")
                 event.set_results(action_results)
 
@@ -5645,11 +5648,18 @@ def test_run_action(self):
         assert out.results == {}
         assert self.harness.charm.simple_was_called
 
+    def test_fail_action_no_message(self):
+        with pytest.raises(ops.testing.ActionFailed) as excinfo:
+            self.harness.run_action('fail', {'empty-failure-message': True})
+            assert 'called `fail()`' in str(excinfo.value)
+        assert excinfo.value.message == ''
+
     def test_fail_action(self):
         self._action_results.clear()
         self._action_results["partial"] = "foo"
         with pytest.raises(ops.testing.ActionFailed) as excinfo:
             self.harness.run_action("fail")
+            assert "something went wrong" in str(excinfo.value)
         assert excinfo.value.message == "something went wrong"
         assert excinfo.value.output.logs == ["some progress", "more progress"]
         assert excinfo.value.output.results == {"partial": "foo"}