Hi Vinay,

Thanks for the explanation!  I'm good with changing the PR.  I do have one question though.

You wrote -
> but I forgot that record.exc_text should also be zapped, as it should always reflect the contents of record.exc_info.

Based on the 'always reflect' statement, I'm wondering if there should be a change to `format()` to add an else to the record.exc_info check:
        if record.exc_info:
            if not record.exc_text:
                record.exc_text = self.formatException(record.exc_info)
        else:
            record.exc_text = None
  
Or if should just be in the `prepare()`.

I hope to have the new PR sometime today.  I think the cutoff for 3.7.1 was yesterday, but if you want to try to get this in to that release feel free to make changes.