@classmethod<EOL><INDENT>def _constructor(cls, mapping, configuration):<DEDENT>
raise NotImplementedError("<STR_LIT>")<EOL>
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