[Python-ideas] Potential PEP: with/except

[Nathaniel Smith]

The first concern that comes to my mind is... When I see:

with:
    ...
except:
    ...

Is that a shorthand for

try:
    with:
        ...
except:
    ...

or for

with:
    try:
        ...
    except:
        ...

? Both are plausible, and it makes a big difference, because 'with' already
has an implicit 'except' block built in.

-n

On Tue, Jan 22, 2019, 12:12 Paul Ferrell <pflarr at gmail.com wrote:

> I've found that almost any time I'm writing a 'with' block, it's doing
> something that could throw an exception. As a result, each of those
> 'with' blocks needs to be nested within a 'try' block. Due to the
> nature of 'with', it is rarely (if ever) the case that the try block
> contains anything other than the with block itself.
>
> As a result, I would like to propose that the syntax for 'with' blocks
> be changed such that they can be accompanied by 'except', 'finally',
> and/or 'else' blocks as per a standard 'try' block. These would handle
> exceptions that occur in the 'with' block, including the execution of
> the applicable __enter__ and __exit__ methods.
>
> Example:
>
> try:
>     with open(path) as myfile:
>       ...   # Do stuff with file
> except (OSError, IOError) as err:
>     logger.error("Failed to read/open file {}: {}".format(path, err)
>
> The above would turn into simply:
>
> with open(path) as myfile:
>     ... # Do stuff with file
> except (OSError, IOError) as err:
>     logger.error(...)
>
>
> I think this is rather straightforward in meaning and easy to read,
> and simplifies some unnecessary nesting. I see this as the natural
> evolution of what 'with'
> is all about - replacing necessary try-finally blocks with something
> more elegant. We just didn't include the 'except' portion.
>
> I'm a bit hesitant to put this out there. I'm not worried about it
> getting shot down - that's kind of the point here. I'm just pretty
> strongly against to unnecessary syntactical additions to the language.
> This though, I think I can except. It introduces no new concepts and
> requires no special knowledge to use. There's no question about what
> is going on when you read it.
>
> --
> Paul Ferrell
> pflarr at gmail.com
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