code: def field2default(self, field):
          
          ret = {}
          if "doc_default" in field.metadata:
              ret["default"] = field.metadata["doc_default"]
          else:
              default = field.missing
              if default is not marshmallow.missing and not callable(default):
                  ret["default"] = default
  
          return ret
docs: Return the dictionary containing the field's default value
  
          Will first look for a `doc_default` key in the field's metadata and then
          fall back on the field's `missing` parameter. A callable passed to the
          field's missing parameter will be ignored.
  
          :param Field field: A marshmallow field.
          :rtype: dict