[Python-Dev] Requesting pronouncement on PEP 0424

[Guido van Rossum]

If you give my second patch an LGTM I'll submit it and you're done.

On Mon, Jul 30, 2012 at 9:58 AM, Alex Gaynor <alex.gaynor at gmail.com> wrote:
>
>
> On Mon, Jul 30, 2012 at 9:51 AM, Guido van Rossum <guido at python.org> wrote:
>>
>> Also, I have a few content quibbles:
>>
>> - Is it really worth flagging a negative return value with ValueError?
>> I'd just as well clip this to zero. What's the worry? That the
>> computed value is wrong? But it's only meant to be a hint, and why
>> would -1 be any more wrong than e.g. 1000000000?
>>
>
> This was done for consistency with len(), I'm not particularly attached to
> any behavior.
>
>>
>> - Did you mean to define operator.length_hint()?
>>
>
> Of course :)
>
>>
>> - The default can be zero with no semantic impact, so I think there's
>> no need to require the caller to specify a default.
>>
>
> I suppose that's fair.
>
>>
>> - Most importantly: calling len(obj) and catching TypeError can only
>> be a substitute for the real implementation, which IMO ought to check
>> for the presence of a tp_len slot. Alas, checking hasattr(obj,
>> '__len__') doesn't quite cut it either, since this returns true for a
>> class object that defines a __len__ method for its instances (the
>> class itself doesn't have a length). Still, I worry that calling
>> len(obj) and catching all TypeErrors overspecifies the desired
>> behavior; what I *want* to happen is to check if there is a __len__
>> method, and if so, call it and let any exceptions bubble through. It
>> may be best to add a comment explaining that am implementation doesn't
>> have to follow the letter of the Python code in the PEP, in
>> particular, if obj *has* a __len__() method but calling it raises an
>> exception, then length_hint(obj) may (ought to?) pass this exception
>> on instead of calling obj.__length_hint__().
>>
>
> Seems reasonable, rather than try to spec that out precisely in the
> pseudocode (aka Python ;)) a note like you suggest sounds good.
>
>>
>> --
>> --Guido van Rossum (python.org/~guido)
>
>
>
> Alex
>
> --
> "I disapprove of what you say, but I will defend to the death your right to
> say it." -- Evelyn Beatrice Hall (summarizing Voltaire)
> "The people's good is the highest law." -- Cicero
>



-- 
--Guido van Rossum (python.org/~guido)