[Python-Dev] __length_hint__

Benjamin Peterson benjamin at python.org
Fri Apr 3 01:17:08 CEST 2009

2009/4/2 Daniel Stutzbach <daniel at stutzbachenterprises.com>:
> Iterators can implement a method called __length_hint__ that provides a hint
> to certain internal routines (such as list.extend) so they can operate more
> efficiently.  As far as I can tell, __length_hint__ is currently
> undocumented.  Should it be?

This has been discussed, and no, it is a implementation detail mostly
for the optimization of builtin iterators.

> If so, are there any constraints on what an iterator should return?  I can
> think of 3 possible rules, each with advantages and disadvantages:
> 1. return your best guess
> 2. return your best guess that you are certain is not higher than the true
> value
> 3. return your best guess that you are certain is not lower than the true
> value
> Also, I've noticed that if a VERY large hint is returned by the iterator,
> list.extend will sometimes disregard the hint and try to allocate memory
> incrementally (correct for rule #1 or #2).  However, in another code path it
> will throw a MemoryError immediately based on the hint (correct for rule
> #3).

Perhaps Raymond can shed some light on these.


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