It seems noone liked (or remembered) an idea I proposed last february, but I'm going to repost it anyway:
How about adding a "rebinding" operator, for example spelled ":=":
a := 2
It would mean: bind the value 2 to the nearest scope that defines 'a'.
A better summary by someone else who liked it: http://groups.google.com/groups?selm=mailman.1048248875.10571.python- list%40python.org
Advantages: no declarative statement (I don't like global much to begin with, but much less for scope declarations other that what it means now). It's a nice addition to the current scoping rule: an assignment IS a scope declaration.
Possible disadvantage: you can only rebind to the nearest scope that defines the name. If there's a farther scope that also defines that name you can't reach that. But that's nicely symmetrical with how _reading_ values from nested scopes work today, shadowing is nothing new.
Ideally, augmented assignments would also become "rebinding". However, this may have compatibility problems.