code: def list_inputs(self):
          
          doc = []
          for inp, typ in self.input_types.items():
              if isinstance(typ, six.string_types):
                  typ = "'{}'".format(typ)
              doc.append('{}: {}'.format(inp, typ))
          return '\n'.join(doc)
docs: Return a string listing all the Step's input names and their types.
  
          The types are returned in a copy/pastable format, so if the type is
          `string`, `'string'` (with single quotes) is returned.
  
          Returns:
              str containing all input names and types.