itertools.product比嵌套循环慢 [英] itertools.product slower than nested for loops
问题描述
我正在尝试使用itertools.product函数使我的代码段(在同位素模式模拟器中)更易于阅读，并且希望也更快(解决方案
您原来的itertool代码在不必要的lambda上花费了很多额外的时间，并手动构建了中间值列表-其中很多可以替换为内置功能.</p>
现在，内部for循环确实增加了很多额外的开销:只需尝试以下操作，其性能与原始代码相当:
for a in itertools.product(carbons,hydrogens,nitrogens,oxygens17,
oxygens18,sulfurs33,sulfurs34,sulfurs36):
i, j, k, l, m, n, o, p = a
totals.append((i[0]+j[0]+k[0]+l[0]+m[0]+n[0]+o[0]+p[0],
i[1]*j[1]*k[1]*l[1]*m[1]*n[1]*o[1]*p[1]))
以下代码在CPython内置方面尽可能多地运行，并且我对其进行了测试，使其等效于代码.值得注意的是，代码使用zip(*iterable)解压缩每个产品结果.然后将reduce与operator.mul用作乘积，并对sum进行求和； 2个用于查看列表的生成器. for循环仍然略胜一筹，但从长远来看，它可能不是您可以使用的硬编码.
import itertools
from operator import mul
from functools import partial
prod = partial(reduce, mul)
elems = carbons, hydrogens, nitrogens, oxygens17, oxygens18, sulfurs33, sulfurs34, sulfurs36
p = itertools.product(*elems)
totals = [
( sum(massdiffs), prod(chances) )
for massdiffs, chances in
( zip(*i) for i in p )
]
I am trying using the itertools.product function to make a segment of my code (in an isotopic pattern simulator) easier to read and hopefully faster as well (the documentation states that no intermediate results are created) , I have however tested both versions of the code against each other using the cProfiling library and noticed that the itertools.product was significantly slower than my nested for loops.
Example values used for the testing:
carbons = [(0.0, 0.004613223957020534), (1.00335, 0.02494768843632857), (2.0067, 0.0673219412049374), (3.0100499999999997, 0.12087054681917497), (4.0134, 0.16243239687902825), (5.01675, 0.17427700732161705), (6.020099999999999, 0.15550695260604208), (7.0234499999999995, 0.11869556397525197), (8.0268, 0.07911287899598853), (9.030149999999999, 0.04677626606764402)]
hydrogens = [(0.0, 0.9417611429667746), (1.00628, 0.05651245007201512)]
nitrogens = [(0.0, 0.16148864310897554), (0.99703, 0.2949830688288726), (1.99406, 0.26887643366755537), (2.99109, 0.16305943261399866), (3.98812, 0.0740163089529218), (4.98515, 0.026824040474519875), (5.98218, 0.008084687617425748)]
oxygens17 = [(0.0, 0.8269292736927519), (1.00422, 0.15717628899143962), (2.00844, 0.014907548827832968)]
oxygens18 = [(0.0, 0.3584191873916266), (2.00425, 0.36813434247849824), (4.0085, 0.18867830334103902), (6.01275, 0.06433912182670033), (8.017, 0.016421642936302827)]
sulfurs33 = [(0.0, 0.02204843659673093), (0.99939, 0.08442569434459646), (1.99878, 0.16131398792444965), (2.99817, 0.2050722764666321), (3.99756, 0.1951327596407101), (4.99695, 0.14824112268069747), (5.99634, 0.09365899226198841), (6.99573, 0.050618028523695714), (7.99512, 0.023888506307006133), (8.99451, 0.010000884811585533)]
sulfurs34 = [(0.0, 3.0106350597190195e-10), (1.9958, 6.747270089956428e-09), (3.9916, 7.54568412614702e-08), (5.9874, 5.614443102700176e-07), (7.9832, 3.1268212758750728e-06), (9.979, 1.3903197959791067e-05), (11.9748, 5.141248916434075e-05), (13.970600000000001, 0.0001626288218672788), (15.9664, 0.00044921518047309414), (17.9622, 0.0011007203440032396)]
sulfurs36 = [(0.0, 0.904828368500412), (3.99501, 0.0905009370374487)]
Snippet demonstrating nested for loops:
totals = []
for i in carbons:
for j in hydrogens:
for k in nitrogens:
for l in oxygens17:
for m in oxygens18:
for n in sulfurs33:
for o in sulfurs34:
for p in sulfurs36:
totals.append((i[0]+j[0]+k[0]+l[0]+m[0]+n[0]+o[0]+p[0], i[1]*j[1]*k[1]*l[1]*m[1]*n[1]*o[1]*p[1]))
Snippet demonstrating the use of itertools.product:
totals = []
for i in itertools.product(carbons,hydrogens,nitrogens,oxygens17,oxygens18,sulfurs33,sulfurs34,sulfurs36):
massDiff = i[0][0]
chance = i[0][1]
for j in i[1:]:
massDiff += j[0]
chance = chance * j[1]
totals.append((massDiff,chance))
The results from profiling (based on 10 runs per method) was an average of ~0.8 seconds for the nested for loop approach and ~1.3 seconds for the itertools.product approach. My question is thus, am I using the itertools.product function wrongly or should I just stick to the nested for loops?
-- UPDATE --
I have included two of my cProfile results:
# ITERTOOLS.PRODUCT APPROACH
420003 function calls in 1.306 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.018 0.018 1.306 1.306 <string>:1(<module>)
1 1.246 1.246 1.289 1.289 IsotopeBas.py:64(option1)
420000 0.042 0.000 0.042 0.000 {method 'append' of 'list' objects}
1 0.000 0.000 0.000 0.000 {method 'disable' of '_lsprof.Profiler' objects}
and:
# NESTED FOR LOOP APPROACH
420003 function calls in 0.830 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.019 0.019 0.830 0.830 <string>:1(<module>)
1 0.769 0.769 0.811 0.811 IsotopeBas.py:78(option2)
420000 0.042 0.000 0.042 0.000 {method 'append' of 'list' objects}
1 0.000 0.000 0.000 0.000 {method 'disable' of '_lsprof.Profiler' objects}
Your original itertool code spent a lot extra time in the needless lambda, and building lists of intermediate values by hand - a lot of this can be replaced with builtin functionality.
Now, the inner for loop does add quite a lot extra overhead: just try the following and the performance is very much on par with your original code:
for a in itertools.product(carbons,hydrogens,nitrogens,oxygens17,
oxygens18,sulfurs33,sulfurs34,sulfurs36):
i, j, k, l, m, n, o, p = a
totals.append((i[0]+j[0]+k[0]+l[0]+m[0]+n[0]+o[0]+p[0],
i[1]*j[1]*k[1]*l[1]*m[1]*n[1]*o[1]*p[1]))
The following code runs as much as possible in the CPython builtin side, and I tested it to be equivalent to with code. Notably the code uses zip(*iterable) to unzip each of the product results; then uses the reduce with operator.mul for product, and sum for summing; 2 generators for going through the lists. The for loop still beats slightly, but being hardcoded it probably is not what you can use in the long run.
import itertools
from operator import mul
from functools import partial
prod = partial(reduce, mul)
elems = carbons, hydrogens, nitrogens, oxygens17, oxygens18, sulfurs33, sulfurs34, sulfurs36
p = itertools.product(*elems)
totals = [
( sum(massdiffs), prod(chances) )
for massdiffs, chances in
( zip(*i) for i in p )
]
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