Handle Logger.exception() outside "except" block

Fixes: hynek#634

src/structlog/processors.py

@@ -28,6 +28,8 @@
     NamedTuple,
     Sequence,
     TextIO,
+    Tuple,
+    Union,
 )
 
 from ._frames import (
@@ -407,11 +409,9 @@ def __init__(
     def __call__(
         self, logger: WrappedLogger, name: str, event_dict: EventDict
     ) -> EventDict:
-        exc_info = event_dict.pop("exc_info", None)
-        if exc_info:
-            event_dict["exception"] = self.format_exception(
-                _figure_out_exc_info(exc_info)
-            )
+        exc_info = _figure_out_exc_info(event_dict.pop("exc_info", None))
+        if exc_info != (None, None, None):
+            event_dict["exception"] = self.format_exception(exc_info)
 
         return event_dict
 
@@ -586,7 +586,7 @@ def __call__(
         return event_dict
 
 
-def _figure_out_exc_info(v: Any) -> ExcInfo:
+def _figure_out_exc_info(v: Any) -> Union[ExcInfo, Tuple[None, None, None]]:
     """
     Depending on the Python version will try to do the smartest thing possible
     to transform *v* into an ``exc_info`` tuple.
@@ -598,7 +598,8 @@ def _figure_out_exc_info(v: Any) -> ExcInfo:
         return v
 
     if v:
-        return sys.exc_info()  # type: ignore[return-value]
+        # Can be "(None, None, None)" if not called inside an error context:
+        return sys.exc_info()
 
     return v
 
@@ -642,7 +643,7 @@ def __call__(
         exc = event_dict.pop("exception", None)
         if exc is None:
             exc_info = _figure_out_exc_info(event_dict.pop("exc_info", None))
-            if exc_info:
+            if exc_info != (None, None, None):
                 exc = _format_exception(exc_info)
 
         if exc:

tests/test_tracebacks.py

@@ -15,6 +15,8 @@
 
 import pytest
 
+import structlog
+
 from structlog import tracebacks
 
 
@@ -741,3 +743,74 @@ def test_exception_dict_transformer_missing_exc_info():
     result = transformer(exc_info=(None, None, None))
 
     assert [] == result
+
+
+class TestLogException:
+    """
+    Higher level "integration tests" for "Logger.exception()".
+    """
+
+    @pytest.fixture()
+    def cap_logs(self) -> structlog.testing.LogCapture:
+        """
+        Create a LogCapture to be used as processor and fixture for retrieving
+        logs in tests.
+        """
+        return structlog.testing.LogCapture()
+
+    @pytest.fixture()
+    def logger(
+        self, cap_logs: structlog.testing.LogCapture
+    ) -> structlog.Logger:
+        """
+        Create a logger with the dict_tracebacks and a LogCapture processor.
+        """
+        old_processors = structlog.get_config()["processors"]
+        structlog.configure([structlog.processors.dict_tracebacks, cap_logs])
+        logger = structlog.get_logger("dict_tracebacks")
+        try:
+            yield logger
+        finally:
+            structlog.configure(processors=old_processors)
+
+    def test_log_explicit_exception(
+        self, logger: structlog.Logger, cap_logs: structlog.testing.LogCapture
+    ) -> None:
+        """
+        The log row contains a traceback when "Logger.exception()" is
+        explicitly called with an exception instance.
+        """
+        try:
+            1 / 0
+        except ZeroDivisionError as e:
+            logger.exception("onoes", exception=e)
+
+        log_row = cap_logs.entries[0]
+        assert log_row["exception"][0]["exc_type"] == "ZeroDivisionError"
+
+    def test_log_implicit_exception(
+        self, logger: structlog.Logger, cap_logs: structlog.testing.LogCapture
+    ) -> None:
+        """
+        The log row contains a traceback when "Logger.exception()" is called
+        in an "except" block but without explicitly passing the exception.
+        """
+        try:
+            1 / 0
+        except ZeroDivisionError:
+            logger.exception("onoes")
+
+        log_row = cap_logs.entries[0]
+        assert log_row["exception"][0]["exc_type"] == "ZeroDivisionError"
+
+    def test_no_exception(
+        self, logger: structlog.Logger, cap_logs: structlog.testing.LogCapture
+    ) -> None:
+        """
+        "Logger.exception()" should not be called outside an "except" block
+        but this cases is gracefully handled and does not lead to an exception.
+
+        See: https://github.com/hynek/structlog/issues/634
+        """
+        logger.exception("onoes")
+        assert [{"event": "onoes", "log_level": "error"}] == cap_logs.entries