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Not all exceptions occurring during SyslogHandler's emit(..) method are caught and handled since the try-except block only covers the latter part of the method.
Thus, since handleError is not invoked, such exceptions might cause the whole program to crash, even though logging.raiseExceptions is unset.
Execution example:
In [1]: import sys
In [2]: sys.version
Out[2]: '2.7.6 (default, Mar 22 2014, 22:59:56) \n[GCC 4.8.2]'
In [3]: import logging.handlers
In [4]: logging.raiseExceptions = 0
In [5]: syslogHandler = logging.handlers.SysLogHandler()
In [6]: logger = logging.getLogger()
In [7]: logger.addHandler(syslogHandler)
In [8]: logger.critical('1', '1')
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-8-3d1c89a066c4> in <module>()
----> 1 logger.critical('1', '1')
/usr/lib/python2.7/logging/__init__.pyc in critical(self, msg, *args, **kwargs)
 1195 """
 1196 if self.isEnabledFor(CRITICAL):
-> 1197 self._log(CRITICAL, msg, args, **kwargs)
 1198 
 1199 fatal = critical
/usr/lib/python2.7/logging/__init__.pyc in _log(self, level, msg, args, exc_info, extra)
 1269 exc_info = sys.exc_info()
 1270 record = self.makeRecord(self.name, level, fn, lno, msg, args, exc_info, func, extra)
-> 1271 self.handle(record)
 1272 
 1273 def handle(self, record):
/usr/lib/python2.7/logging/__init__.pyc in handle(self, record)
 1279 """
 1280 if (not self.disabled) and self.filter(record):
-> 1281 self.callHandlers(record)
 1282 
 1283 def addHandler(self, hdlr):
/usr/lib/python2.7/logging/__init__.pyc in callHandlers(self, record)
 1319 found = found + 1
 1320 if record.levelno >= hdlr.level:
-> 1321 hdlr.handle(record)
 1322 if not c.propagate:
 1323 c = None #break out
/usr/lib/python2.7/logging/__init__.pyc in handle(self, record)
 747 self.acquire()
 748 try:
--> 749 self.emit(record)
 750 finally:
 751 self.release()
/usr/lib/python2.7/logging/handlers.pyc in emit(self, record)
 840 exception information is present, it is NOT sent to the server.
 841 """
--> 842 msg = self.format(record) + '000円'
 843 """
 844 We need to convert record level to lowercase, maybe this will
/usr/lib/python2.7/logging/__init__.pyc in format(self, record)
 722 else:
 723 fmt = _defaultFormatter
--> 724 return fmt.format(record)
 725 
 726 def emit(self, record):
/usr/lib/python2.7/logging/__init__.pyc in format(self, record)
 462 it is formatted using formatException() and appended to the message.
 463 """
--> 464 record.message = record.getMessage()
 465 if self.usesTime():
 466 record.asctime = self.formatTime(record, self.datefmt)
/usr/lib/python2.7/logging/__init__.pyc in getMessage(self)
 326 msg = self.msg #Defer encoding till later
 327 if self.args:
--> 328 msg = msg % self.args
 329 return msg
 330 
TypeError: not all arguments converted during string formatting
In [9]: 
Executing similar code via StreamHandler for example, doesn't raise the TypeError exception.

New changeset 54549f9b2ecc by Vinay Sajip in branch 'default':
Closes #22776: Merged fix from 3.4.
https://hg.python.org/cpython/rev/54549f9b2ecc 

N.B. 2.7 and 3.4 also updated, but I accidentally left the issue no. out of the commit message. Relevant changesets are ea7b64406396 and f6a906541476.