[Python-Dev] Do we need length_hint at all? (Was PEP 0424: A method for exposing a length hint) (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon Jul 16 16🔞55 CEST 2012

On Tue, Jul 17, 2012 at 12:01 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:

On Mon, 16 Jul 2012 23:23:18 +1000 Nick Coghlan <ncoghlan at gmail.com> wrote:

- distinguishing between different reasons for saying "don't preallocate any space" (i.e. returning zero). I still haven't heard a convincing rationale for this one The point is that zero is a valid value for a length hint. By making it a special value for "don't know", you are making the protocol potentially confusing, and you are also departing from the current semantics.

No, it just means the default estimate is always zero. If you don't do that, then every client of length_hint has to check for the magic value. It's making the API harder to use for no good reason.

(and, yes, I think distinguishing between zero and "don't know" is useful: imagine a container that would preallocate 256 entries by default when the answer is "don't know")

Such a container has to already deal with the case when it overestimates severely. The only cost of using zero as a default estimate is that such hypothetical containers will overallocate when they technically didn't need to, which will already happen for any empty iterator that doesn't provide length_hint at all.

Given that all standard library containers default to assuming a size of zero (absent information indicating otherwise), and a special value would need to be special cased by every client of the API (and almost always defaulted to zero), it's simply not a good trade-off.

The PEP itself already has this general tone, but I think that it should be even more emphatic that it's about codifying the status quo, not about modifying it with ideas haven't been proven useful through past experience. Then the PEP shouldn't address infinite iterators at all.

Noting that infinite iterators are free to define length_hint to always throw an exception is the status quo. We just haven't done it for the stdlib ones.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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