[Python-Dev] PEP 0424: A method for exposing a length hint

Thu Aug 2 17:43:38 CEST 2012

On Aug 1, 2012, at 1:46 AM, Mark Shannon wrote:
>> '''
> Being able to pre-allocate lists based on the expected size, as estimated by __length_hint__,
> can be a significant optimization.
> PyPy has been observed to run some code slower than CPython, purely because this optimization is absent.
> '''
>> Which is a PyPy bug report, not a rationale for a PEP ;)

Alex's rationale is correct and well expressed.
Your proposed revision reflects fuzzy thinking about why __length_hint__ is useful.
Regardless of resizing growth factors, it is *always* helpful to know how much
memory to allocate. Calls to the allocators (especially for large blocks)
and possible the recopying of data should be avoided when possible.
Raymond
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