[Python-Dev] PEP 0424: A method for exposing a length hint (original) (raw)

Alex Gaynor alex.gaynor at gmail.com
Sun Jul 15 07:20:03 CEST 2012

On Sat, Jul 14, 2012 at 10:16 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

On Sun, Jul 15, 2012 at 9:18 AM, Benjamin Peterson <benjamin at python.org> wrote: >> Open questions >> ============== >> >> There are two open questions for this PEP: >> >> * Should list expose a kwarg in it's constructor for supplying a length >> hint. >> * Should a function be added either to builtins or some other module which >> calls _lengthhint_, like builtins.len calls _len_. > > Let's try to keep this as limited as possible for a public API.

Length hints are very useful for any container implementation, whether those containers are in the standard library or not. Just as we exposed operator.index when index was added, we should expose an "operator.lengthhint" function with the following semantics: def lengthhint(obj): """Return an estimate of the number of items in obj. This is useful for presizing containers when building from an iterable. If the object supports len(), the result will be exact. Otherwise, it may over or underestimate by an arbitrary amount. The result will be an integer >= 0. """ try: return len(obj) except TypeError: try: gethint = obj.lengthhint except AttributeError: return 0 hint = gethint() if not isinstance(hint, int): raise TypeError("Length hint must be an integer, not %r" % type(hint)) if hint < 0:_ _raise ValueError("Length hint (%r) must be >= 0" % hint) return hint There's no reason to make pure Python container implementations reimplement all that for themselves. Cheers, Nick. -- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

Sounds reasonable to me, the only issue with your psuedocode (err... I mean Python ;)), is that there's no way for the lenght_hint to specify that that particular instance can't have a length hint computed. e.g. imagine some sort of lazy stream that cached itself, and only wanted to offer a length hint if it had already been evaluated. Without an exception to raise, it has to return whatever the magic value for length_hint is (in your impl it appears to be 0, the current _PyObject_LengthHint method in CPython has a required default parameter). The PEP proposes using TypeError for that.

Anyways that code looks good, do you want to add it to the PEP?

Alex

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