[Python-Dev] __length_hint__

Daniel Stutzbach daniel at stutzbachenterprises.com
Fri Apr 3 01:14:59 CEST 2009

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?

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
3. return your best guess that you are certain is not lower than the true

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

Daniel Stutzbach, Ph.D.
President, Stutzbach Enterprises, LLC <http://stutzbachenterprises.com>
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