Fix: prevent PositionalArgumentsFormatter from crashing on TypeError (#258)

src/structlog/stdlib.py

@@ -12,6 +12,7 @@
 from __future__ import annotations
 
 import asyncio
+import contextlib
 import contextvars
 import functools
 import logging
@@ -802,7 +803,8 @@ def __call__(
             if len(args) == 1 and isinstance(args[0], dict) and args[0]:
                 args = args[0]
 
-            event_dict["event"] %= args
+            with contextlib.suppress(TypeError, ValueError):
+                event_dict["event"] %= args
 
         if self.remove_positional_args and args is not None:
             del event_dict["positional_args"]

tests/test_stdlib.py

@@ -518,6 +518,27 @@ def test_args_removed_if_empty(self):
 
         assert {} == formatter(None, None, {"positional_args": ()})
 
+    def test_graceful_failure_on_formatting_error(self):
+        """
+        If positional arguments do not match the formatting placeholders
+        in the event string, PositionalArgumentsFormatter should not crash
+        with a TypeError or ValueError. It should leave the event string
+        unformatted.
+
+        Regression test for https://github.com/hynek/structlog/issues/258.
+        """
+        formatter = PositionalArgumentsFormatter()
+
+        # Mismatched placeholders (event has no %s but args are passed)
+        event_dict = formatter(
+            None,
+            None,
+            {"event": "Info message", "positional_args": ("x",)},
+        )
+
+        assert "Info message" == event_dict["event"]
+        assert "positional_args" not in event_dict
+
 
 class TestAddLogLevelNumber:
     @pytest.mark.parametrize(("level", "number"), NAME_TO_LEVEL.items())