fix: format traceback inline for loguru compatibility (#622)

Stop passing `exc_info=` as a kwarg to `logger.log()`. Standard
`logging.Logger` treats this specially (appends traceback), but loguru
interprets all kwargs as message format parameters, causing a KeyError.

Instead, format the traceback into the message string directly using
`traceback.format_exception()`. This works with all loggers that
implement `LoggerProtocol` (logging, loguru, structlog, etc.).

Also tighten `LoggerProtocol.log()` by removing `**kwargs` since we
no longer pass any keyword arguments.

Fixes #420


Change-Id: Icc145539d443c83f09266e25b9819bd2516aeddc

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: jd

tenacity/_utils.py

@@ -32,9 +32,7 @@ class LoggerProtocol(typing.Protocol):
     Compatible with logging, structlog, loguru, etc...
     """
 
-    def log(
-        self, level: int, msg: str, *args: typing.Any, **kwargs: typing.Any
-    ) -> typing.Any: ...
+    def log(self, level: int, msg: str, *args: typing.Any) -> typing.Any: ...
 
 
 def find_ordinal(pos_num: int) -> str:

tenacity/before_sleep.py

@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import traceback
 import typing
 
 from tenacity import _utils
@@ -35,8 +36,6 @@ def before_sleep_log(
     """Before sleep strategy that logs to some logger the attempt."""
 
     def log_it(retry_state: "RetryCallState") -> None:
-        local_exc_info: BaseException | bool | None
-
         if retry_state.outcome is None:
             raise RuntimeError("log_it() called before outcome was set")
 
@@ -46,22 +45,22 @@ def log_it(retry_state: "RetryCallState") -> None:
         if retry_state.outcome.failed:
             ex = retry_state.outcome.exception()
             verb, value = "raised", f"{ex.__class__.__name__}: {ex}"
-
-            if exc_info:
-                local_exc_info = retry_state.outcome.exception()
-            else:
-                local_exc_info = False
         else:
             verb, value = "returned", retry_state.outcome.result()
-            local_exc_info = False  # exc_info does not apply when no exception
 
         fn_name = retry_state.get_fn_name()
 
-        logger.log(
-            log_level,
+        msg = (
             f"Retrying {fn_name} "
-            f"in {sec_format % retry_state.next_action.sleep} seconds as it {verb} {value}.",
-            exc_info=local_exc_info,
+            f"in {sec_format % retry_state.next_action.sleep} seconds as it {verb} {value}."
         )
 
+        if exc_info and retry_state.outcome.failed:
+            ex = retry_state.outcome.exception()
+            if ex is not None:
+                tb = "".join(traceback.format_exception(type(ex), ex, ex.__traceback__))
+                msg = f"{msg}\n{tb.rstrip()}"
+
+        logger.log(log_level, msg)
+
     return log_it