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What does exc_info do in a logging call?

Roy Smith roy at panix.com
Thu Feb 23 09:21:25 EST 2012 
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?
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