When completing a simple identifier, it completes keywords, built-ins
and globals in __main__; when completing NAME.NAME..., it evaluates
(!) the expression up to the last dot and completes its attributes.

It's very cool to do "import string" type "string.", hit the
completion key (twice), and see the list of names defined by the
string module!

Tip: to use the tab key as the completion key, call

    readline.parse_and_bind("tab: complete")