Question re local bindings in nested scopes

[Wolfgang Lipp]

On Mon, 04 Jun 2001 09:55:41 -0700, Kirby Urner <urner at alumni.princeton.edu> wrote:
> 
> I don't get why the following two functions, each with internal
> functions that take advantage of lexical scoping (new in 2.1),
> behave so differently:
> 
>   from __future__ import nested_scopes
> 
>   def spam(a,b):
>       def bar():
>           output = []
>           print b
>           print a
>           output.append(a+1)
>           return output
>       return [x for x in bar()]
> 
>   def foo(a,b):
>       def bar():
>           output = []
>           print b
>           print a
>           a += 1
>           output.append(a)
>           return output
>       return [x for x in bar()]
> 
>   >>> spam(1,10)
>   10
>   1
>   [2]
>   >>> foo(1,10)
>   10
>   Traceback (most recent call last):
>     File "<pyshell#16>", line 1, in ?
>       foo(1,10)
>     File "d:\program files\python21\ocn\test.py", line 38, in foo
>       return [x for x in bar()]
>     File "d:\program files\python21\ocn\test.py", line 34, in bar
>       print a
>   UnboundLocalError: local variable 'a' referenced before assignment
> 
> I can fix foo() by assigning a to s and doing the increment on s:
> 
>   def foo(a,b):
>       def bar():
>           output = []
>           print b
>           print a
>           s = a   # added this line
>           s += 1 # added this line
>           output.append(s)
>           return output
>       return [x for x in bar()]
> 
> But should a=a+1 (or a += 1) really break the initial binding of a 
> to the outer scope?  Is this intended?  Given a and b initialize to
> the outer scope values just fine in spam(), it seems odd to unbind
> an already initialized variable at "compile time" just because we 
> add 1 to it.  Seems too subtle for its own good.  a=a+1 would seem 
> a runtime operation on a variable that is already initialized at 
> compile time thanks to lexical scoping.
> 
> Kirby
> 
> CC replies/insights to pdx4d at teleport.com and/or above address please
>