[Python-ideas] except expression

[MRAB]

On 2014-02-18 01:30, Ethan Furman wrote:
> 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.
>
IMHO, if "pass" were allowed, it should be as part of a statement, not
an expression, so:

     menu.remove(mint) except ValueError: pass

would be short for:

     try:
         menu.remove(mint)
     except ValueError:
         pass

but:

     func(menu.remove(mint) except ValueError: pass)

wouldn't be allowed.