Second wild idea of the day:
The dict constructor currently accepts sequences where each element has length 2, interpreted as a key-value pair.
Let's have it also accept sequences with elements of length 1, interpreted as a key:None pair.
The benefit is that it provides a way to rapidly construct sets:
lowercase = dict('abcdefghijklmnopqrstuvwxyz') if char in lowercase: ...
dict([key1, key2, key3, key1]).keys() # eliminate duplicate keys
Rejecting (even in the modified form you showed after prompring from Tim). I think the dict() constructor is already overloaded to the brim. Let's do a set module instead. There's only one hurdle to take for a set module, and that's the issue with using mutable sets as keys. Let's just pick one solution and implement it (my favorite being that sets simply cannot be used as keys, since it's the simplest, and matches dicts and lists).
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)