[Python-Dev] PEP 463: Exception-catching expressions

[Ethan Furman]

On 02/23/2014 11:26 AM, Thomas Wouters wrote:
>
> On Feb 23, 2014 7:52 PM, "Stefan Behnel" <stefan_ml at behnel.de <mailto:stefan_ml at behnel.de>> wrote:
>>
>> Chris Angelico, 21.02.2014 04:15:
>> > Just as PEP 308 introduced a means of value-based conditions in an
>> > expression, this system allows exception-based conditions to be used
>> > as part of an expression.
>> > [...]
>> > This currently works::
>> >
>> >     lst = [1, 2, None, 3]
>> >     value = lst[2] or "No value"
>> >
>> > The proposal adds this::
>> >
>> >     lst = [1, 2]
>> >     value = lst[2] except IndexError: "No value"
>>
>> I see a risk of interfering with in-place assignment operators, e.g.
>>
>>     x /= y except ZeroDivisionError: 1
>>
>> might not do what one could expect, because (as I assume) it would behave
>> differently from
>>
>>     x = x / y except ZeroDivisionError: 1
>
> Yes. Augmented assignment is still assignment, so a statement. The only way to parse that is as
>
> x /= (y except ZeroDivisionError: 1)

Well, that is certainly not what I would have expected.

I can also see how parentheses can help, but I still would like them optional.

--
~Ethan~