code: def map_to_openapi_type(self, *args):
          
          if len(args) == 1 and args[0] in self.field_mapping:
              openapi_type_field = self.field_mapping[args[0]]
          elif len(args) == 2:
              openapi_type_field = args
          else:
              raise TypeError("Pass core marshmallow field type or (type, fmt) pair.")
  
          def inner(field_type):
              self.field_mapping[field_type] = openapi_type_field
              return field_type
  
          return inner
docs: Decorator to set mapping for custom fields.
  
          ``*args`` can be:
  
          - a pair of the form ``(type, format)``
          - a core marshmallow field type (in which case we reuse that type's mapping)