fix: format traceback inline for loguru compatibility

[jd]

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

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

[Delgan]

Hi @jd. Thanks for considering updating tenacity to support loguru.

The change looks good. I tested it locally and I confirmed that the @retry decorator can now be used successfully with Loguru's logger.

[mergify]

Merge Queue Status

Rule: default

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• ✅ Entered queue — 2026-03-05 07:49 UTC
• ✅ Checks passed · on draft merge queue: embarking main (e104116) and #622 together #624
• ✅ Merged — 2026-03-05 07:49 UTC · at fa76be48562050c531fa2370a65060ac7fec9e31

This pull request spent 11 seconds in the queue, including 1 second running CI.

Required conditions to merge

[Copilot]

Pull request overview

This PR addresses issue #420 by fixing a KeyError that occurred when using loguru (or similar loggers) with before_sleep_log(). The root cause was that the old code passed exc_info= as a keyword argument to logger.log(), which standard logging.Logger handles specially (appending a traceback), but loguru treats as a message format parameter, causing a KeyError. The fix embeds the traceback directly into the message string using traceback.format_exception(), making the behavior portable across all loggers conforming to LoggerProtocol.

Changes:

• The traceback is now formatted inline into the log message string using traceback.format_exception() instead of being passed as exc_info= kwarg to logger.log()
• The local_exc_info intermediate variable and the exc_info= kwarg in logger.log() calls are removed
• LoggerProtocol.log() has **kwargs removed from its signature

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tenacity/before_sleep.py	Replaces exc_info= kwarg in logger.log() with inline traceback formatting into the message string
tenacity/_utils.py	Removes **kwargs from LoggerProtocol.log() since it is no longer called with keyword arguments

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[Copilot]

The ex variable is already assigned at line 46 when retry_state.outcome.failed is True. Since the guard condition on line 58 (if exc_info and retry_state.outcome.failed) ensures we are in the same failed branch, the re-assignment ex = retry_state.outcome.exception() on line 59 is a redundant duplicate call. The previously set ex from line 46 can be reused directly, eliminating the extra method call.

Suggested change

ex = retry_state.outcome.exception()

[Copilot]

Removing **kwargs from LoggerProtocol.log() is a breaking API change for any users who have custom logger implementations typed against this protocol. The previous signature was def log(self, level: int, msg: str, *args: Any, **kwargs: Any) -> Any, which matched the broader logging.Logger.log signature that accepts stacklevel, extra, etc. as keyword arguments. Users who previously passed such keyword arguments through a LoggerProtocol-typed reference will now get a static type error. Consider whether a more conservative change (keeping **kwargs in the protocol even if tenacity itself no longer passes kwargs) would be preferable for backward compatibility.

Suggested change

	def log(self, level: int, msg: str, *args: typing.Any) -> typing.Any: ...
	def log(
	self, level: int, msg: str, *args: typing.Any, **kwargs: typing.Any
	) -> typing.Any: ...