Adjust showwarning, not formatwarning.

ops/log.py

@@ -23,11 +23,6 @@
 from ops.model import _ModelBackend
 
 
-# We do this on module import because some warnings are issued before we set up
-# the framework, and we need to capture those as well.
-logging.captureWarnings(True)
-
-
 class JujuLogHandler(logging.Handler):
     """A handler for sending logs and warnings to Juju via juju-log."""
 
@@ -65,18 +60,19 @@ def setup_root_logging(
     logger = logging.getLogger()
     logger.setLevel(logging.DEBUG)
     logger.addHandler(JujuLogHandler(model_backend))
+    logging.captureWarnings(True)
 
-    def custom_warning_formatter(
+    def custom_showwarning(
         message: typing.Union[str, Warning],
         category: typing.Type[Warning],
         filename: str,
         lineno: int,
-        _: typing.Optional[str] = None,
+        *_: typing.Any,
     ) -> str:
-        """Like the default formatter, but don't include the code."""
+        """Like the default showwarning, but don't include the code."""
         return f'{filename}:{lineno}: {category.__name__}: {message}'
 
-    warnings.formatwarning = custom_warning_formatter
+    warnings.showwarning = custom_showwarning
 
     if debug:
         handler = logging.StreamHandler()

testing/src/scenario/_ops_main_mock.py

@@ -7,6 +7,7 @@
 import marshal
 import re
 import sys
+import warnings
 from typing import TYPE_CHECKING, Any, Dict, FrozenSet, List, Sequence, Set
 
 import ops
@@ -137,12 +138,17 @@ def _load_charm_meta(self):
         return ops.CharmMeta.from_yaml(metadata, actions_metadata)
 
     def _setup_root_logging(self):
-        # Ops sets sys.excepthook to go to Juju's debug-log, but that's not
-        # useful in a testing context, so we reset it here.
+        # The warnings module captures this in _showwarning_orig, but we
+        # shouldn't really be using a private method, so capture it ourselves as
+        # well.
+        original_showwarning = warnings.showwarning
         super()._setup_root_logging()
         # Ops also sets up logging to capture warnings, but we want the normal
         # output.
         logging.captureWarnings(False)
+        warnings.showwarning = original_showwarning
+        # Ops sets sys.excepthook to go to Juju's debug-log, but that's not
+        # useful in a testing context, so we reset it here.
         sys.excepthook = sys.__excepthook__
 
     def _make_storage(self, _: _Dispatcher):