Add type casting to fix failing mypy lint

[samuelallan72]

The mypy lints were failing since the release of ops 2.17.0. The problem does not appear to be with ops,
but rather with how type narrowing works with mypy, and how the charm assigns the unit status in different methods. See canonical/operator#1401 for more context.

We can fix the issue with no changes to the charm logic by adding type casting to undo the type narrowing in this case.

[samuelallan72]

Note: ideally we would refactor the charm to avoid setting unit.status from many locations - instead hooking into the collect-status event and setting the unit status from one method. But this is a minimal change to unblock the CI, until we have time to refactor the code.

[dimaqq]

Drive-by 2c:

The proposed cast is [semantically] equivalent to # type: ignore.

I'd rather see a type ignore with a comment like Status collection across XXX needs to be refactored, a band-aid for now and an issue opened to eventually refactor.

[chanchiwai-ray]

@dimaqq was mentioning about using type ignore instead, what do you think? For me, it makes more sense to ignore a typing error than hacking the code to make the type checker happy

[samuelallan72]

The proposed cast is [semantically] equivalent to # type: ignore.

I'm not sure how this is equivalent? Wouldn't ignoring the type be more equivalent to casting it to Any? By type casting to StatusBase, we're asserting to the type checker that it's at least compatible with StatusBase, which should allow it to provide us more useful information than simply ignoring the type altogether.

[samuelallan72]

Note, if we went the route of ignoring the type, we'd need the ignore comment on all the lines affected by the "unreachable" rule. Eg. the first one would look like this:

diff --git a/src/charm.py b/src/charm.py
index 029bda9..f594fa6 100755
--- a/src/charm.py
+++ b/src/charm.py
@@ -163,8 +163,8 @@ class StorageConnectorCharm(CharmBase):
             return
 
         self._check_mandatory_config()
-        if isinstance(self.unit.status, BlockedStatus):
-            return
+        if isinstance(self.unit.status, BlockedStatus):  # type: ignore
+            return  # type: ignore
 
         # install packages
         cache = apt.cache.Cache()