[Python-Dev] Requesting pronouncement on PEP 0424

Alex Gaynor alex.gaynor at gmail.com
Mon Jul 30 18:58:21 CEST 2012

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)


"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
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