docs: def _fix_up_fields(cls):
          """Add names to all of the Endpoint's Arguments.
  
          This method will get called on class declaration because of
          Endpoint's metaclass.  The functionality is based on Google's NDB
          implementation."""
          cls._arguments = dict()
          if cls.__module__ == __name__: # skip the classes in this file
              return
          for name in set(dir(cls)):
              attr = getattr(cls, name, None)
              if isinstance(attr, BaseArgument):
                  if name.startswith('_'):
                      raise TypeError("Endpoint argument %s cannot begin with "
                              "an underscore, as these attributes are reserved "
                              "for instance variables of the endpoint object, "
                              "rather than for arguments to your HTTP Endpoint."
                              % name)
                  attr._fix_up(cls, name)
                  cls._arguments[attr.name] = attr