[Python-Dev] PEP for Better Control of Nested Lexical Scopes

[Mark Russell]

On 21 Feb 2006, at 19:25, Jeremy Hylton wrote:
> If I recall the discussion correctly, Guido said he was open to a
> version of nested scopes that allowed rebinding.

PEP 227 mentions using := as a rebinding operator, but rejects the  
idea as it would encourage the use of closures.  But to me it seems  
more elegant than some special keyword, especially is it could also  
replace the "global" keyword.  It doesn't handle things like "x += y"  
but I think you could deal with that by just writing "x := x + y".

BTW I do think there are some cases where replacing a closure with a  
class is not an improvement.  For example (and assuming the existence  
of :=):

     def check_items(items):
         had_error = False

         def err(mesg):
             print mesg
             had_error := True

         for item in items:
             if too_big(item):
                 err("Too big")
             if too_small(item):
                 err("Too small")

         if had_error:
             print "Some items were out of range"

Using a class for this kind of trivial bookkeeping just adds  
boilerplate and obscures the main purpose of the code:

     def check_items(items):
         class NoteErrors (object):
             def __init__(self):
                 self.had_error = False

             def __call__(self, mesg):
                 print mesg
                 self.had_error = True

         err = NoteErrors()

         for item in items:
             if too_big(item):
                 err("Too big")
             if too_small(item):
                 err("Too small")

         if err.had_error:
             print "Some items were out of range"

Any chance of := (and removing "global") in python 3K?

Mark Russell