What does exc_info do in a logging call?

[Peter Otten]

Roy Smith wrote:

> In http://docs.python.org/release/2.6.7/library/logging.html, it says:
> 
> logging.debug(msg[, *args[, **kwargs]])
> [...]
> There are two keyword arguments in kwargs which are inspected: exc_info
> which, if it does not evaluate as false, causes exception information to
> be added to the logging message. If an exception tuple (in the format
> returned by sys.exc_info()) is provided, it is used; otherwise,
> sys.exc_info() is called to get the exception information.
> 
> I don't get what this is trying to do.  I have this code:
> 
>         try:
>             f = urllib2.urlopen(url)
>         except urllib2.HTTPError as ex:
>             logger.error("Unable to retrieve profile from facebook
> (access_token='%r')" % access_token,
>                          exc_info=ex)
> 
> which ends up logging:
> 
> [...] ERROR _get_profile Unable to retrieve profile from facebook
> (access_token='u'[token elided]'')
> 
> so what is the exc_info doing?  What "exception information" is being
> added to the message?  Am I just calling this wrong?

I think whatever the formatter's formatException() makes out of the 
(exception_type, exception_value, traceback) tuple lands in the logfile:

>> import logging
>>> logging.basicConfig()
>>> try: 1/0
... except: logging.error("yadda")
...
ERROR:root:yadda
>>> try: 1/0
... except: logging.error("yadda", exc_info=True)
...
ERROR:root:yadda
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
>>> class Formatter(logging.Formatter):
...     def formatException(self, *args): return "OOPS"
...
>>> logging._handlers.keys()[0].setFormatter(Formatter()) # ;)
>>> try: 1/0
... except: logging.error("yadda", exc_info=True)
...
yadda
OOPS