=============
      defaultdict
     =============
     
     The standard dictionary includes the method :func:`setdefault` for
     retrieving a value and establishing a default if the value does not
     exist. By contrast, :class:`defaultdict` lets the caller specify the
     default up front when the container is initialized.
     
     .. include:: collections_defaultdict.py
         :literal:
         :start-after: #end_pymotw_header
     
     This works well as long as it is appropriate for all keys to have the
     same default. It can be especially useful if the default is a type
     used for aggregating or accumulating values, such as a :class:`list`,
     :class:`set`, or even :class:`int`. The standard library documentation
     includes several examples of using :class:`defaultdict` this way.
     
     .. {{{cog
     .. cog.out(run_script(cog.inFile, 'collections_defaultdict.py'))
     .. }}}
     
     ::
     
     	$ python collections_defaultdict.py
     
     	d: defaultdict(<function default_factory at 0x10045ba28>, {'foo': 'bar'})
     	foo => bar
     	bar => default value
     
     .. {{{end}}}
     
     .. seealso::
     
         `defaultdict examples <http://docs.python.org/lib/defaultdict-examples.html>`_
             Examples of using defaultdict from the standard library documentation.
     
         `James Tauber: Evolution of Default Dictionaries in Python <http://jtauber.com/blog/2008/02/27/evolution_of_default_dictionaries_in_python/>`_
             Discussion of how defaultdict relates to other means of initializing dictionaries.