def find_unknown_references_hook(config):
  """Hook to find/raise errors for references to unknown configurables."""
  additional_msg_fmt = " In binding for '{}'."
  for (scope, selector), param_bindings in six.iteritems(config):
    for param_name, param_value in six.iteritems(param_bindings):
      for maybe_unknown in _iterate_flattened_values(param_value):
        if isinstance(maybe_unknown, _UnknownConfigurableReference):
          scope_str = scope + '/' if scope else ''
          min_selector = _REGISTRY.minimal_selector(selector)
          binding_key = '{}{}.{}'.format(scope_str, min_selector, param_name)
          additional_msg = additional_msg_fmt.format(binding_key)
          _raise_unknown_reference_error(maybe_unknown, additional_msg)
Hook to find/raise errors for references to unknown configurables.