[Python-Dev] anonymous blocks

[Steven Bethard]

James Y Knight wrote:
> If it was possible to assign to a variable to a variable bound outside
> your function, but still in your lexical scope, I think it would fix
> this issue. That's always something I've thought should be possible,
> anyways. I propose to make it possible via a declaration similar to
> 'global'.
> 
> E.g. (stupid example, but it demonstrates the syntax):
> def f():
>    count = 0
>    def addCount():
>      lexical count
>      count += 1
>    assert count == 0
>    addCount()
>    assert count == 1

It strikes me that with something like this lexical declaration, we
could abuse decorators as per Carl Banks's recipe[1] to get the
equivalent of thunks:

def withfile(filename, mode='r'):
    def _(func):
        f = open(filename, mode)
        try:
            func(f)
        finally:
            f.close()
    return _

and used like:

line = None
@withfile("readme.txt")
def print_readme(fileobj):
    lexical line
    for line in fileobj:
        print line
print "last line:" line

As the recipe notes, the main difference between print_readme and a
real "code block" is that print_readme doesn't have access to the
lexical scope.  Something like James's suggestion would solve this
problem.

One advantage I see of this route (i.e. using defs + lexical scoping
instead of new syntactic support) is that because we're using a normal
function, the parameter list is not an issue -- arguments to the
"thunk" are bound to names just as they are in any other function.

The big disadvantage I see is that my normal expectations for
decorators are wrong here -- after the decorator is applied
print_readme is set to None, not a new callable object.

Guess I'm still riding the fence. ;-)

STeVe

[1]http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/391199
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