Load a serializer.

def get_serializer(serializer):
    """ Load a serializer. """
    if isinstance(serializer, string_types):
        try:
            app_label, serializer_name = serializer.split('.')
            app_package = get_application(app_label)
            serializer_module = import_module('%s.serializers' % app_package)
            serializer = getattr(serializer_module, serializer_name)
        except Exception as e:
            logger.error('Serializer %s not found: %s' % (serializer, e))
            return None
    return serializer