def __call__(self, key):
if key not in self:<EOL><INDENT>raise AttributeError(<EOL>"<STR_LIT>".format(<EOL>cls=self.__class__.__name__, name=key<EOL>)<EOL>)<EOL><DEDENT>return self._build(self[key])<EOL>
Dynamically access a key-value pair.

key: A key associated with a value in the mapping.

This differs from __getitem__, because it returns a new instance
of an Attr (if the value is a Mapping object).