[Python-ideas] except expression

[Ethan Furman]

On 02/17/2014 04:57 PM, MRAB wrote:
> On 2014-02-18 00:20, Ethan Furman wrote:
>> On 02/17/2014 03:54 PM, Chris Angelico wrote:
>>> On Tue, Feb 18, 2014 at 9:26 AM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>>>> There's no logical difference between not generating
>>>> a result at all, and generating a result of None and
>>>> then throwing it away.
>>>>
>>>> The same thing applies here:
>>>>
>>>>     menu.remove(mint) except ValueError: pass
>>>>
>>>> This says exactly what the programmer means: "Remove
>>>> mint from the menu if it's there, otherwise do nothing."
>>>
>>> In a statement context, it's possible to say "otherwise do nothing".
>>> An if without an else does this, as does putting 'pass' in certain
>>> places.
>>>
>>> But in an expression context, there are 'two possibility'. Either some
>>> kind of value is returned, or an exception is raised. (At least, I
>>> don't think there are any other options. Harry Hoo, not the Spanish
>>> Inquisition.) What should happen here:
>>>
>>> func(menu.remove(mint) except ValueError: pass)
>>>
>>> If remove() raises ValueError, should func be called? If so, with what
>>> argument? (Note that it's because English is happy with "passing"
>>> arguments to functions that I was happy with the notation "except
>>> ValueError pass None", but that's using pass as a syntactic element,
>>> in place of the colon.)
>>
>> Yes, func should be called, and it should be called with no arguments.
>>
> In that case, is:
>
>      func(foo if condition else pass)
>
> allowed (being like "func(foo) if condition else func()")?
>
> And, in fact, is:
>
>      func(pass)
>
> also allowed (meaning "func()")?

I'm not saying it's a good idea, just that it's the natural conclusion from allowing pass.

I don't care for it, myself.  Just use None, since that's Python "No value" value.

--
~Ethan~