docs: def build_swagger12_handler(schema):
      """Builds a swagger12 handler or returns None if no schema is present.
  
      :type schema: :class:`pyramid_swagger.model.SwaggerSchema`
      :rtype: :class:`SwaggerHandler` or None
      """
      if schema:
          return SwaggerHandler(
              op_for_request=schema.validators_for_request,
              handle_request=handle_request,
              handle_response=validate_response,
          )