Return operation log format.

def _get_log_format(self, request):
        """Return operation log format."""
        user = getattr(request, 'user', None)
        if not user:
            return
        if not request.user.is_authenticated:
            return
        method = request.method.upper()
        if not (method in self.target_methods):
            return
        request_url = urlparse.unquote(request.path)
        for rule in self._ignored_urls:
            if rule.search(request_url):
                return
        return self.format