Message 65742 - Python tracker (original) (raw)

On Thu, Apr 24, 2008 at 2:23 PM, John Arbash Meinel <report@bugs.python.org> wrote: ..

So if you compare consuming a generator multiple times to creating it each time, it is 0.662 usec - 0.173 usec = 0.489 usec to create a generator.

So why does: "(i for i in l); x.update(y)" take an additional 1.208 usec.

(I'm certainly willing to believe that set.update() is generator/list agnostic, but something weird is still happening.)

I've seen a similar strangeness in timings:

$ python -m timeit '(i for i in [])' 100000 loops, best of 3: 4.16 usec per loop

but

$ python -m timeit -s 'x = set()' 'x.update(i for i in [])' 1000000 loops, best of 3: 1.31 usec per loop

on the other hand,

$ python -m timeit -s 'x = []' 'x.extend(i for i in [])' 100000 loops, best of 3: 4.54 usec per loop

How can x.update(i for i in []) take less time than simply creating a genexp?

Note that there is no apparent bytecode tricks here:

1 0 LOAD_CONST 0 (<code object at 0xf7e88920, file "", line 1>) 3 MAKE_FUNCTION 0 6 BUILD_LIST 0 9 GET_ITER 10 CALL_FUNCTION 1 13 RETURN_VALUE

dis(lambda:x.update(i for i in [])) 1 0 LOAD_GLOBAL 0 (x) 3 LOAD_ATTR 1 (update) 6 LOAD_CONST 0 (<code object at 0xf7e88920, file "", line 1>) 9 MAKE_FUNCTION 0 12 BUILD_LIST 0 15 GET_ITER 16 CALL_FUNCTION 1 19 CALL_FUNCTION 1 22 RETURN_VALUE