[Python-ideas] Potential PEP: with/except

[Abe Dillon]

[Calvin Spealman]

> Why not allow excepts on fo loops, for example?


Why not, indeed...

I've heard there's a non-insignificant performance penalty for setting up a
try statement, so it might be important to only set a for-loop up as a
guarded for-loop upon reading the "except" statement (if the compiler can
handle such behavior).

On Tue, Jan 22, 2019 at 2:24 PM Calvin Spealman <cspealma at redhat.com> wrote:

>
>
> On Tue, Jan 22, 2019 at 3:11 PM 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(...)
>>
>>
> It definitely makes sense, both the problem and the proposed solution.
>
> The thing that concerns me is that any such problem and solution seems
> to apply equally to any other kind of block. Why not allow excepts on fo
> loops, for example?
>
>
>>
>> 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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>>
>
>
> --
>
> CALVIN SPEALMAN
>
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