[Python-ideas] with-except-finally blocks

[Ryan Gonzalez]

Is that any more ambiguous than for...else?


On Thu, Apr 16, 2015 at 2:22 AM, Andrew Barnert <
abarnert at yahoo.com.dmarc.invalid> wrote:

> On Apr 15, 2015, at 23:39, Kale Kundert <kale at thekunderts.net> wrote:
> >
> > Hello everyone,
> >
> > I'd like to propose a little bit of syntactic sugar: allowing
> with-blocks to be
> > followed by except- and finally-blocks just like try-blocks.  For
> example:
> >
> > with open('spam.txt') as file:
> >    print(file.read())
> > except IOError:
> >    print('No spam here...')
> > finally:
> >    print('done.')
> >
> > This proposed syntax is semantically equivalent to wrapping a with-block
> within
> > a try-block like so:
> >
> > try:
> >    with open('spam.txt') as file:
> >        print(file.read())
> >    finally:
> >        print('done.')
> > except IOError:
> >    print('No spam here...')
>
> Why does the finally come before the except? Also, it's still indented
> with the with, which means you seem to be defining with-except-finally in
> terms of with-finally, and not defining with-finally at all. Did you mean
> to have the finally after the except and dedented?
>
> Anyway, I like the idea, but does the grammar work? A with that has an
> optional except and finally at the same level seems potentially ambiguous.
> For example:
>
>     with spam:
>         pass
>     with eggs:
>         pass
>     finally:
>         pass
>
> If you think about it, it's pretty obvious that the finally goes with the
> second with, not the first. But can the parser tell that? Or a human who's
> scanning the code rather than thinking carefully about it? Or tools like
> auto-indenters?
>
> (I think this problem doesn't arise for try, because it must have at least
> except or finally.)
>
> It should be easy to edit the grammar and build the parser to at least the
> first part of the question; for the last part, trying to hack up a few
> different external tools like Emacs python-mode. But for the human
> question, I'm not sure how to answer it quite so easily...
>
>
> >
> > I see two advantages to the proposed syntax.  First and most obviously,
> it saves
> > an extra line and an extra indentation level.  One line may not be a big
> deal,
> > but one indentation level can really affect readability.  Second and more
> > conceptually, it makes sense to think about exception handling in the
> context of
> > a with-block.  More often than not, if you're using a with-block, you're
> > expecting that something in that block could throw an exception.  Usually
> > with-blocks are used to make sure resources (e.g. files, database
> sessions,
> > mutexes, etc.) are properly closed before the exception is propogated.
> But I
> > very often want to do some custom clean-up as well (alert the user,
> etc.).
> > Currently that requires wrapping the whole thing in a try-block, but
> allowing
> > with-blocks to behave as try-blocks is a more direct way to express what
> is meant.
> >
> > I was curious how often with-blocks are actually wrapped in try-blocks
> for no
> > other purpose than catching exceptions raised in the with-block.  So I
> searched
> > through a number of open source projects looking (roughly) for that
> pattern:
> >
> > Project         with [1]  try-with [2]
> > ==============  ========  ============
> > django          230       17
> > ipython         541       8
> > matplotlib      112       3
> > moinmoin        10        0
> > numpy           166       1
> > pillow/pil      1         0
> > pypy            254       4
> > scipy           163       2
> > sqlalchemy      36        0
> > twisted         72        1
> > ==============  ========  ============
> > total           1585      36 (2.27%)
> >
> > [1]: grep -Po '^\s*with .*:' **/*.py
> > [2]: grep -Poz 'try:\s*with .*:' **/*.py
> >
> > Assuming these projects are representative, about 2% of the with-blocks
> are
> > directly wrapped by try-blocks.  That's not a huge proportion, but it
> clearly
> > shows that this pattern is being used "in the wild".  Whether or not
> it's worth
> > changing the language for the benefit of 2% of with-blocks is something
> to
> > debate though.
> >
> > What do people think of this idea?
> >
> > -Kale Kundert
> >
> > _______________________________________________
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-- 
Ryan
[ERROR]: Your autotools build scripts are 200 lines longer than your
program. Something’s wrong.
http://kirbyfan64.github.io/
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