def get_field(self, field_name):
try:<EOL><INDENT>return self._fields[self.app.resolve_field_name(field_name)]<EOL><DEDENT>except KeyError:<EOL><INDENT>raise UnknownField(self.app, field_name, self._fields.keys())<EOL><DEDENT>
Get field instance used to get, set, and serialize internal field value

        Args:
            field_name (str): Field name or key to retrieve

        Returns:
            Field: Requested field instance

        Raises:
             UnknownField: Raised if `field_name` not found in parent App