[Python-Dev] Merging PEP 310 and PEP 340-redux?

[Alex Martelli]

On May 9, 2005, at 21:58, Guido van Rossum wrote:

> Apologies if this has been discovered and rejected already; I've had
> to skip most of the discussions but this though won't leave my head...

Skimming rather than skipping all of the discussion burned most of my  
py-dev time, and it was just skimming, but I don't remember such  
rejections.

> But what if we changed the translation slightly so that VAR gets
> assigned to value of the __enter__() call:
>
>         abc = EXPR
>         VAR = abc.__enter__()      # I don't see why it should be  
> optional
>         try:
>             BLOCK
>         finally:
>             abc.__exit__()
>
> Now it would make more sense to change the syntax to
>
>         with EXPR as VAR:
>             BLOCK
>
> and we have Phillip Eby's proposal. The advantage of this is that you

I like this.  The only aspect of other proposals that I would sorely  
miss here, would be the inability for abc.__exit__ to deal with  
exceptions raised in BLOCK (or, even better, a separate specialmethod  
on abc called in lieu of __exit__ upon exceptions).  Or am I missing  
something, and would this give a way within abc.__exit__ to examine  
and possibly ``unraise'' such an exception...?

> can write a relatively straightforward decorator, call it
> @with_template, that endows a generator with the __enter__ and
> __exit__ methods, so you can write all the examples (except
> auto_retry(), which was there mostly to make a point) from PEP 340
> like this:
>
>         @with_template
>         def opening(filename, mode="r"):
>             f = open(filename, mode)
>             yield f
>             f.close()
>
> and so on. (Note the absence of a try/finally block in the generator
> -- the try/finally is guaranteed by the with-statement but not by the
> generator framework.)

I must be thick this morning, because this relatively straightforward  
decorator isn't immediately obvious to me -- care to show me how  
with_template gets coded?


Alex