"with" statement in python, why must the "as" section be a single object

Question

With Python one can say:

a,b,c=something_that_returns_a_3_tuple()

But a with statement like:

class thing(object):
    def __enter__(self):
        return (1,2,3)

    def __exit__(self,a,b,c):
        pass

with thing() as a,b,c:
    print a
    print b 
    print c

will not work work

One must have:

class thing(object):
    def __enter__(self):
        return (1,2,3)

    def __exit__(self,a,b,c):
        pass

with thing() as (a,b,c):
    print a
    print b 
    print c

I can't see a practical problem with allowing the first form, by this I mean implementation or logical, the parser shouldn't have a problem with commas (it wont be ambiguous) and I see no logical reason why not.

http://docs.python.org/release/2.5/whatsnew/pep-343.html

and http://www.python.org/dev/peps/pep-0343/ suggests this is just syntatic sugar

Answer 1

You cannot use a tuple target without parenthesis in the syntax, because otherwise you cannot distinguish between multiple context managers:

with contextmanager1 as target1, contextmanager2 as target2:

See the comma there? Because the grammar allows for specifying multiple context managers, the compiler cannot distinguish between that form and multiple targets specified with just commas.

Ah, you say, but there is an as target2 part there, can't that be used to distinguish the forms? Then remember, that the target is optional. The following is perfectly legal syntax too:

with contextmanager1 as target1, contextmanager2:

Python knows the part after the comma is another context manager there, because it disallowed for multiple targets without parenthesis, removing all ambiguity.

Support for multiple context managers was introduced after PEP 343 was implemented, in Python 2.7 and 3.1. However, the PEP did anticipate the possibility:

This restriction makes a future extension possible of the syntax to have multiple comma-separated resources, each with its own optional as-clause.