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I needed two logging handlers in my application, one notifiying the user
of errors, the other writing errors to a logfile. So I created a custom
subclass of logging.Formatter and redefined the formatException() method
that returned a summary of the exception like 'ZeroDivisionError'
instead of the full traceback.
Unfortunately the logging record caches the result of the
handler.formatException() call in the exc_text variable, and the
formatException() method of the second handler isn't called at all.
The attached patch removes the caching and fixes the problem.

This is tough. On the one hand you are right that different classes that
have different formatException() methods aren't treated correctly; on
the other hand I think the caching is important for other cases where
there are multiple loggers all using the default formatException().
I recommend that instead of getting rid of the cache, you fix your class
by overriding format() to reset record.ex c_text and then calling the
base class format() method.

> > This is tough. On the one hand you are right that different classes that
> > have different formatException() methods aren't treated correctly; on
> > the other hand I think the caching is important for other cases where
> > there are multiple loggers all using the default formatException().
> > 
> > I recommend that instead of getting rid of the cache, you fix your class
> > by overriding format() to reset record.ex c_text and then calling the
> > base class format() method.
That is what I have been doing although I think that I have to reset
record.exc_text also AFTER the call to format() for the following
handlers, if any.
So, should this issue be closed as 'wont fix' ?

I think so.
On 10/18/07, Thomas Heller <report@bugs.python.org> wrote:
>
> Thomas Heller added the comment:
>
> > > This is tough. On the one hand you are right that different classes that
> > > have different formatException() methods aren't treated correctly; on
> > > the other hand I think the caching is important for other cases where
> > > there are multiple loggers all using the default formatException().
> > >
> > > I recommend that instead of getting rid of the cache, you fix your class
> > > by overriding format() to reset record.ex c_text and then calling the
> > > base class format() method.
>
> That is what I have been doing although I think that I have to reset
> record.exc_text also AFTER the call to format() for the following
> handlers, if any.
>
> So, should this issue be closed as 'wont fix' ?
>
> __________________________________
> Tracker <report@bugs.python.org>
> <http://bugs.python.org/issue1295>
> __________________________________
>

I think that a warning or an example in the docs would be nice, but I
have no time to make a patch for that.

I've updated the docs (Formatter.format) to mention the caching and its
effect on multiple formatters.