[Python-ideas] except expression

[MRAB]

On 2014-02-13 12:50, Nick Coghlan wrote:
> On 13 February 2014 20:10, M.-A. Lemburg <mal at egenix.com> wrote:
>> On 13.02.2014 10:24, Nick Coghlan wrote:
>>> General comment: like Raymond, I'm inclined to favour a nice expression
>>> friendly exception handling syntax, precisely because of the proliferation
>>> of relatively ad hoc alternative solutions (in particular, the popularity
>>> of being able to pass in default values to handle empty iterables).
>>
>> Here's a variant the resembles the code you'd write in a helper
>> function to achieve the same thing, only stripped down somewhat:
>>
>> x = something() except ValueError return default_value
>>
>> def try_something():
>>     try:
>>         return something()
>>     except ValueError:
>>         return default_value
>>
>> x = something() except ValueError as exc return exc.message
>>
>> def try_something():
>>     try:
>>         return something()
>>     except ValueError as exc
>>         return exc.message
>>
>> Obviously, having a keyword "use" would make a better fit :-)
>>
>> x = something() except ValueError use default_value
>
> Even if we don't agree on a resolution for 3.5, I think there's more
> than enough interest for it to be worth someone's while to collate
> some of the proposals in a PEP - if nothing else, it will save
> rehashing the whole discussion next time it comes up :)
>
> The benefits of this:
>
> - the status quo is that various APIs are growing "default" parameters
> to handle the case where they would otherwise throw an exception
> - this is creating inconsistencies, as some such functions can be used
> easily as expressions without risking any exception (those where such
> a parameter has been added), as well as a temptation to use "Look
> Before You Leap" pre-checks, even in cases where exception handling
> would be a better choice
> - sequence indexing is a case where there is no current standard
> mechanism for providing a default value, so you're either use a
> pre-check for the system length, or else using a full try statement or
> context manager to handle the IndexError
> - by providing a clean, expression level syntax for handling a single
> except clause and providing an alternate value for the expression,
> this problem could be solved once and for all in a systematic way,
> rather than needing to incrementally change the API of a variety of
> functions (as well as addressing the container subscripting case in a
> way that doesn't require switching away from using the subscript
> syntax to a normal function call, or switching from use an expression
> to a statement)
>
>
> Some of the specific syntactic proposals:
>
>      x = op() except default if Exception
>      x = op() except default for Exception
>      x = op() except default from Exception
>      x = op() except Exception return default
>
>      x = op() except exc.attr if Exception as exc
>      x = op() except exc.attr for Exception as exc
>      x = op() except exc.attr from Exception as exc
>      x = op() except Exception as exc return exc.attr
>
You left one out:

      x = op() except Exception: default