[Python-Dev] Requesting pronouncement on PEP 0424

Guido van Rossum guido at python.org
Mon Jul 30 18:51:09 CEST 2012

On Sun, Jul 29, 2012 at 6:11 PM, Alex Gaynor <alex.gaynor at gmail.com> wrote:
> Guido van Rossum <guido <at> python.org> writes:
>> Looks good to me, so accepted.But why isn't it visible on python.org/dev/peps/
> yet?
> I just realized the text in the python.org repo did not match what I had locally.
> I've pushed what I intended to be the latest text, if everyone could take a new
> look at that I would be very grateful. Sorry for the mixup.


I look a careful look at what's in Hg (still totally different from
what python.org displays) and am proposing a few editorial changes;
please see the review at http://codereview.appspot.com/6447061

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?

- Did you mean to define operator.length_hint()?

- The default can be zero with no semantic impact, so I think there's
no need to require the caller to specify a default.

- 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__().

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

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