[Python-Dev] collections module (original) (raw)

Tim Peters tim.one at comcast.net
Sun Jan 11 19:31:28 EST 2004

[Kurt B. Kaiser]

... If the overall slowdown of lists to get fast queues was 5%, that would no doubt be unacceptable. If it was 0.5%, I imagine it would be acceptable?

Probably not -- it's too much the tail wagging the dog, as Python lists, used for what they're best at, are core language workhorses already. Timing changes under 10% are very hard to establish convincingly, though, especially because the relative performance of Python features varies so widely across platforms (some mix of the code the platform C compiler generates, the quality of the platform C libraries, and pure accidents, like whether the top of the main eval loop happens to end up fighting severe I-cache conflicts with other high-frequency code).

Note that if no pop(0)'s are ever done on the list there is no wrap around and the slowdown is limited to the access of blocksize plus a test; no modulo arithmetic is necessary. This is also the case if the list doesn't grow and items are only removed, including list[0]. And in the latter case, the memmoves associated with internal deletions will dominate the performance in all implementations.

Neither case holds for a queue; the question that started this was whether to provide a fancy queue implementation in C, and (of course) Guido would rather see Python lists efficiently usable for that purpose too. I suspected that might not be a realistic hope, and now I'm starting to think it too .

More information about the Python-Dev mailing list