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

Tue Feb 21 21:41:26 CET 2006

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