Drop event dict keys colliding with LogRecord attributes in render_to_log_*

render_to_log_kwargs() and render_to_log_args_and_kwargs() build the
extra kwarg straight out of whatever is left in the event dict once the
event, positional_args, and the exc_info/stack_info/stacklevel trio have
been extracted. If a remaining key happens to share a name with an
attribute that logging.LogRecord already carries (filename, module,
process, message, ...), logging.Logger.makeRecord() rejects the whole
call with a raw KeyError, crashing a documented, non-ProcessorFormatter
configuration on an entirely plausible business field name.

structlog.stdlib.ExtraAdder already filters LogRecord's own attribute
names out of the event dict when merging a record's extra fields in the
other direction. Apply the same filtering here so a colliding key is
dropped from extra instead of reaching logging.Logger.makeRecord() and
crashing it.

Fixes #486.

CHANGELOG.md

@@ -15,6 +15,11 @@ You can find our backwards-compatibility policy [here](https://github.com/hynek/
 
 ## [Unreleased](https://github.com/hynek/structlog/compare/26.1.0...HEAD)
 
+### Fixed
+
+- `structlog.stdlib.render_to_log_kwargs()` and `structlog.stdlib.render_to_log_args_and_kwargs()` now drop event dict keys that collide with a `logging.LogRecord` attribute (for example `filename` or `module`) instead of crashing the standard library with `KeyError: "Attempt to overwrite '...' in LogRecord"`.
+  [#486](https://github.com/hynek/structlog/issues/486)
+
 
 ## [26.1.0](https://github.com/hynek/structlog/compare/25.5.0...26.1.0) - 2026-06-06
 

src/structlog/stdlib.py

@@ -883,6 +883,14 @@ _LOG_RECORD_KEYS = logging.LogRecord(
     "name", 0, "pathname", 0, "msg", (), None
 ).__dict__.keys()
 
+# `Logger.makeRecord` raises `KeyError` for an `extra` key already on the
+# `LogRecord`: `_LOG_RECORD_KEYS`, plus "message"/"asctime" added later by
+# `Formatter.format()`. An event field named "filename" would crash.
+_RESERVED_LOG_RECORD_KEYS = frozenset(_LOG_RECORD_KEYS) | {
+    "message",
+    "asctime",
+}
+
 
 class ExtraAdder:
     """
@@ -960,9 +968,17 @@ def render_to_log_args_and_kwargs(
     arguments, keyword arguments are extracted from the *event_dict* and the
     rest of the *event_dict* is added as ``extra``.
 
+    Keys that collide with an attribute `logging.LogRecord` already carries
+    (for example ``filename`` or ``module``) are dropped instead of being
+    added to ``extra``, because the standard library rejects them there with
+    a `KeyError` at the point of logging.
+
     This allows you to defer formatting to `logging`.
 
     .. versionadded:: 25.1.0
+    .. versionchanged:: 26.2.0
+       Keys colliding with `logging.LogRecord` attributes are now dropped
+       from ``extra`` instead of crashing the standard library.
     """
     args = (event_dict.pop("event"), *event_dict.pop("positional_args", ()))
 
@@ -971,6 +987,8 @@ def render_to_log_args_and_kwargs(
         for kwarg_name in LOG_KWARG_NAMES
         if kwarg_name in event_dict
     }
+    for key in event_dict.keys() & _RESERVED_LOG_RECORD_KEYS:
+        del event_dict[key]
     if event_dict:
         kwargs["extra"] = event_dict
 
@@ -989,6 +1007,11 @@ def render_to_log_kwargs(
     extracted from the *event_dict* and the rest of the *event_dict* is added as
     ``extra``.
 
+    Keys that collide with an attribute `logging.LogRecord` already carries
+    (for example ``filename`` or ``module``) are dropped instead of being
+    added to ``extra``, because the standard library rejects them there with
+    a `KeyError` at the point of logging.
+
     This allows you to defer formatting to `logging`.
 
     .. versionadded:: 17.1.0
@@ -997,16 +1020,18 @@ def render_to_log_kwargs(
        kwargs and not put into ``extra``.
     .. versionchanged:: 24.2.0
        ``stackLevel`` corrected to ``stacklevel``.
+    .. versionchanged:: 26.2.0
+       Keys colliding with `logging.LogRecord` attributes are now dropped
+       from ``extra`` instead of crashing the standard library.
     """
-    return {
-        "msg": event_dict.pop("event"),
-        "extra": event_dict,
-        **{
-            kw: event_dict.pop(kw)
-            for kw in LOG_KWARG_NAMES
-            if kw in event_dict
-        },
+    msg = event_dict.pop("event")
+    kwargs = {
+        kw: event_dict.pop(kw) for kw in LOG_KWARG_NAMES if kw in event_dict
     }
+    for key in event_dict.keys() & _RESERVED_LOG_RECORD_KEYS:
+        del event_dict[key]
+
+    return {"msg": msg, "extra": event_dict, **kwargs}
 
 
 class ProcessorFormatter(logging.Formatter):

tests/test_stdlib.py

@@ -870,6 +870,34 @@ class TestRenderToLogArgsAndKwargs:
             extra=expected_extra,
         )
 
+    def test_drops_keys_colliding_with_log_record_attributes(
+        self, stdlib_logger: logging.Logger, caplog: pytest.LogCaptureFixture
+    ):
+        """
+        A key that collides with an attribute `logging.LogRecord` already
+        carries (for example "filename") is dropped from `extra` instead of
+        being passed through, because the standard library raises `KeyError`
+        when it's asked to overwrite it while building the record.
+
+        Cf. https://github.com/hynek/structlog/issues/486
+        """
+        event_dict = {
+            "event": "message",
+            "filename": "not-a-real-file.py",
+            "keep": "this",
+        }
+
+        args, kwargs = render_to_log_args_and_kwargs(
+            stdlib_logger, "info", event_dict
+        )
+
+        assert {"extra": {"keep": "this"}} == kwargs
+
+        with caplog.at_level(logging.INFO):
+            stdlib_logger.info(*args, **kwargs)
+
+        assert "this" == caplog.records[0].keep
+
     def test_integration(
         self, stdlib_logger: logging.Logger, event_dict: dict[str, Any]
     ):
@@ -987,6 +1015,41 @@ class TestRenderToLogKwargs:
             logging.INFO, "message", (), **expected
         )
 
+    def test_drops_keys_colliding_with_log_record_attributes(
+        self, stdlib_logger, caplog: pytest.LogCaptureFixture
+    ):
+        """
+        A key that collides with an attribute `logging.LogRecord` already
+        carries (for example "filename") is dropped from `extra` instead of
+        being passed through, because the standard library raises `KeyError`
+        when it's asked to overwrite it while building the record.
+
+        "message" collides too: it isn't on a fresh `LogRecord`, but the
+        standard library still special-cases and rejects it.
+
+        Cf. https://github.com/hynek/structlog/issues/486
+        """
+        d = render_to_log_kwargs(
+            None,
+            None,
+            {
+                "event": "message",
+                "filename": "not-a-real-file.py",
+                "message": "duplicate-of-msg",
+                "keep": "this",
+            },
+        )
+
+        assert {
+            "msg": "message",
+            "extra": {"keep": "this"},
+        } == d
+
+        with caplog.at_level(logging.INFO):
+            stdlib_logger.info(**d)
+
+        assert "this" == caplog.records[0].keep
+
     def test_integration_special_kw(self, event_dict, stdlib_logger):
         """
         render_to_log_kwargs with a wrapped logger calls the stdlib logger