在低长度迭代的限制下考虑以下操作,
d = (3, slice(None, None, None), slice(None, None, None))
In [215]: %timeit any([type(i) == slice for i in d])
1000000 loops, best of 3: 695 ns per loop
In [214]: %timeit any(type(i) == slice for i in d)
1000000 loops, best of 3: 929 ns per loop
设置为list
比使用生成器表达式快 25%?
为什么会出现这种情况?list
是一个额外的操作。
注意:在两次运行中我都收到警告:The slowest run took 6.42 times longer than the fastest. This could mean that an intermediate result is being cached
I
Analysis
在这个特殊的测试中,list()
结构的速度更快,最长可达4
由此发电机的性能得到了提高。
红线显示此事件发生的位置,黑线显示两者性能相同的位置。
The code takes about 1min to run on my MacBook Pro by utilising all the cores:
import timeit, pylab, multiprocessing
import numpy as np
manager = multiprocessing.Manager()
g = manager.list([])
l = manager.list([])
rng = range(1,16) # list lengths
max_series = [3,slice(None, None, None)]*rng[-1] # alternate array types
series = [max_series[:n] for n in rng]
number, reps = 1000000, 5
def func_l(d):
l.append(timeit.repeat("any([type(i) == slice for i in {}])".format(d),repeat=reps, number=number))
print "done List, len:{}".format(len(d))
def func_g(d):
g.append(timeit.repeat("any(type(i) == slice for i in {})".format(d), repeat=reps, number=number))
print "done Generator, len:{}".format(len(d))
p = multiprocessing.Pool(processes=min(16,rng[-1])) # optimize for 16 processors
p.map(func_l, series) # pool list
p.map(func_g, series) # pool gens
ratio = np.asarray(g).mean(axis=1) / np.asarray(l).mean(axis=1)
pylab.plot(rng, ratio, label='av. generator time / av. list time')
pylab.title("{} iterations, averaged over {} runs".format(number,reps))
pylab.xlabel("length of iterable")
pylab.ylabel("Time Ratio (Higher is worse)")
pylab.legend()
lt_zero = np.argmax(ratio<1.)
pylab.axhline(y=1, color='k')
pylab.axvline(x=lt_zero+1, color='r')
pylab.ion() ; pylab.show()