:class:`dict` class when the requested key is not found; whatever it
returns or raises is then returned or raised by :meth:`__getitem__`.
+ Note that :meth:`__missing__` is *not* called for any operations besides
+ :meth:`__getitem__`. This means that :meth:`get` will, like normal
+ dictionaries, return ``None`` as a default rather than using
:class:`defaultdict` objects support the following instance variable: