On Wed, Aug 1, 2012 at 12:06 PM, Mark Shannon <m...@hotpy.org> wrote: > Maciej Fijalkowski wrote: >> >> On Wed, Aug 1, 2012 at 10:46 AM, Mark Shannon <m...@hotpy.org> wrote: >>> >>> While the idea behind PEP 424 is sound, the text of the PEP is rather >>> vague >>> and missing a lot of details. >>> There was extended discussion on the details, but none of that has >>> appeared >>> in the PEP yet. >>> >>> So Alex, how about adding those details? >>> >>> Also the rationale is rather poor. >>> Given that CPython is the reference implementation, PyPy should be >>> compared >>> to CPython, not vice-versa. >>> Reversing PyPy and CPython in the rationale gives: >>> >>> ''' >>> 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 ;) >>> >>> Perhaps a better rationale would something along the lines of: >>> >>> ''' >>> Adding a __length_hint__ method to the iterator protocol allows >>> sequences, >>> notably lists, >>> to be initialised from iterators with only a single resize operation. >>> This allows sequences to be intialised quickly, yet have a small growth >>> factor, reducing memory use. >>> ''' >>> >> >> Hi Mark. >> >> It's not about saving memory. It really is about speed. Noone bothered >> measuring cpython with length hint disabled to compare, however we did >> that for pypy hence the rationale contains it. It's merely to state >> "this seems like an important optimization". Since the C-level code >> involved is rather similar (it's mostly runtime anyway), it seems >> reasonable to draw a conclusion that removing length hint from cpython >> would cause slowdown. > > > It is not about making it faster *or* saving memory, but *both*. > Without __length_hint__ there is a trade off between speed and memory use. > You can have speed at the cost of memory by increasing the resize factor.
No, you cannot. if you allocate a huge region, you're not gonna make much of speed, because at the end you need to copy stuff anyway. Besides large allocations are slow. With length hint that is correct (sometimes you can do that) you have a zero-copy scenario _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
