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

Stefan Behnel stefan_ml at behnel.de
Sun Jul 15 11:11:50 CEST 2012

Alex Gaynor, 15.07.2012 07:20:

there's no way for the lenghthint 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 lengthhint is (in your impl it appears to be 0, the current PyObjectLengthHint method in CPython has a required default parameter). The PEP proposes using TypeError for that.

Yes, that's a major issue. I've been planning to add a length hint to Cython's generator expressions for a while, but the problem is really that in most cases it is only known at runtime if the underlying iterable has a length hint, so propagating it needs a way to say "sorry, I thought I might know, but I don't". It would be even better if this way was efficient. Since we're at a point of making this an official protocol, why not change the current behaviour and return -1 (or even just 0) to explicitly state that "we don't know"?

The problem with an exception here is that it might have been raised accidentally inside of the length_hint() implementation that is being asked. Swallowing it just because it happened to be a TypeError rather than something else may end up covering bugs. We had a similar issue with hasattr() in the past.

Also, it would be nice if this became a type slot rather than requiring a dict lookup and Python function call.

Stefan

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