Guido van Rossum wrote:
[snip illustration of how generators (and other iterators) can be
modified to be used in with-blocks]
> the most common use case should not be an alternate looping syntax
> (even though it *is* technically a loop) but a more general "macro
> statement expansion"
I'm sure I could get used to it, but my intuition for
with f = with_file(filename):
for line in f:
print process(line)
is that the f = with_file(filename) executes only once. That is, as
you said, I don't expect this to be a looping syntax. Of course, as
long as the generators (or other objects) here yield only one value
(like with_file does), then the with-block will execute only once.
But because the implementation lets you make the with-block loop if
you want, it makes be nervous...
I guess it would be helpful to see example where the looping
with-block is useful. So far, I think all the examples I've seen have
been like with_file, which only executes the block once. Of course,
the loop allows you to do anything that you would normally do in a
for-loop, but my feeling is that this is probably better done by
composing a with-block that executes the block only once with a normal
Python for-loop.
I'd almost like to see the with-block translated into something like
it = EXPR
try:
VAR = it.next()
except StopIteration:
raise WithNotStartedException
err = None
try:
BODY
except Exception, err: # Pretend "except Exception:" == "except:"
pass
try:
it.next_ex(err)
except StopIteration:
pass
else:
raise WithNotEndedException
where there is no looping at all, and the iterator is expected to
yield exactly one item and then terminate. Of course this looks a lot
like:
it = EXPR
VAR = it.__enter__()
err = None
try:
BODY
except Exception, err: # Pretend "except Exception:" == "except:"
pass
it.__exit__(err)
So maybe I'm just still stuck on the enter/exit semantics. ;-)
STeVe
--
You can wordify anything if you just verb it.
--- Bucky Katt, Get Fuzzy
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