[Python-Dev] 'With' context documentation draft (was Re: Terminology for PEP 343

[Walter Dörwald]

Guido van Rossum wrote:
> On 7/7/05, Walter Dörwald <walter at livinglogic.de> wrote:
> 
>>What is still unspecified (or at least not explicitely mentioned) in
>>the PEP is the lifetime of VAR in:
>>
>>         with EXPR as VAR:
>>             BLOCK
> 
> It is specified well enough IMO -- you're supposed to take the
> translation into basic Python seriously. That translation specifies a
> simple assignment to VAR, and that's what I meant (and what I'm sure
> most folks understood). IOW VAR lives in the surrounding scope,
> overwrites a previous value, and survives past the with-statement
> (unless it is set inside of course).

Are there use cases for both variants?

If VAR would live only until the end of the with block, this would give 
us a nice way of generating nested data structures:

class blist(list):
    def __init__(self, parent=None):
       if parent:
          parent.append(self)
       list.__init__(self)

    def __enter__(self):
       return self

    def __call__(self, *items):
       self.extend(items)
       return self

x = blist()
x(1)
with blist(x) as x:
    x(2)
    with blist(x) as x:
       x(3)
    x(4)
x(5)

This would create the list:
[1, [2, [3], 4], 5]

With the current version of PEP 343, we would either have to use 
different variable names on each level or use a global stack where 
__enter__() and __exit__() push and pop values.

Bye,
    Walter Dörwald