docs: def _replace_string_args(self):
          '''
          A helper method that makes passing custom validators implemented as
          methods to :class:`incoming.datatypes.Function` instances.
          '''
  
          if self._string_args_replaced:
              return
  
          for field in self._fields:
              field = getattr(self, field)
              if isinstance(field, Function):
                  if isinstance(field.func, str):
                      field.func = getattr(self, field.func)
              elif isinstance(field, JSON):
                  if isinstance(field.cls, str):
                      field.cls = getattr(self, field.cls)
  
          self._string_args_replaced = True