@classmethod
def _constructor(cls, mapping, configuration):
    """
        A standardized constructor used internally by Attr.

        mapping: A mapping of key-value pairs. It is HIGHLY recommended
            that you use this as the internal key-value pair mapping, as
            that will allow nested assignment (e.g., attr.foo.bar = baz)
        configuration: The return value of Attr._configuration
        """
    raise NotImplementedError('You need to implement this')
