docs: def get_identifier(identifier, module_globals, module_name):
      """Helper utility to retrieve the callable function associated with a string identifier.
  
      Args:
          identifier: The identifier. Could be a string or function.
          module_globals: The global objects of the module.
          module_name: The module name
  
      Returns:
          The callable associated with the identifier.
      """
      if isinstance(identifier, six.string_types):
          fn = module_globals.get(identifier)
          if fn is None:
              raise ValueError('Unknown {}: {}'.format(module_name, identifier))
          return fn
      elif callable(identifier):
          return identifier
      else:
          raise ValueError('Could not interpret identifier')