Message2907

Some basic formatting concerns:
  - A space between a parenthesis and a brace.
  - An empty line between all method declarations.
  - Braces on all loops and if/else statements.
  - I'm glad to see you included javadoc, but don't add @param or @return if
    there aren't params or returns or if you're not going to clarify on them.

One basic Jython concern:
  - Never override __getitem__, only __finditem__.

Some general coding concerns:
  - Don't copy code.  It looks like you copied dict_init into dictionary_init
    in defaultdict.  You should've made dict_init callable from there.
  - Don't prepend _ to method names.  We have private in Java.
  - Don't add constructors that don't have an immediate use(in PyDefaultdict).

Overall this seems like a good start, so if you'll fix these little things, I'll actually examine the implementation.