- The copy module now "copies" function objects (as atomic objects).
  - dict.__getitem__ now looks for a __missing__ hook before raising
    KeyError.
  - Added a new type, defaultdict, to the collections module.
    This uses the new __missing__ hook behavior added to dict (see above).

Add this default to UserDict.DictMixin.setdefault() too.

(Championed by Bob Ippolito.)

The update() method for mappings now accepts all the same argument forms
as the dict() constructor.  This includes item lists and/or keyword
arguments.

dictionary as the original.  This parallels MvL's change to
Lib/os.py 1.56.

Backport candidate.

(contributed by Michael Stone.)

Sebastien Keim pointed out that iterkeys and __contains__ require
their own definitions so their behavior will update when the
underlying method is subclassed.

Constructor accepts optional keyword arguments after a optional items list.
Add fromkeys() as an alternate constructor from an iterable over keys.
Expand related unittests.