Python itertools慢吗？

Question

我试图使用Python的itertools模块来加速三重嵌套for循环。下面的测试代码将标准三重嵌套循环与itertools的产品方法和输出进行比较：

嵌套循环时间=2.35023秒

Itertools循环时间=2.67766秒

我错过什么了吗？在

import numpy
import itertools
import time

n = 128
a = numpy.arange(n**3).reshape((n,n,n))
b = numpy.zeros((n,n,n))
c = numpy.zeros((n,n,n))

t = time.time()
for i in range(n):
    for j in range(n):
        for k in range(n):
            b[i,j,k] = a[i,j,k]
print 'Nested loop time = %g secs' % (time.time() - t)

t = time.time()
for (i,j,k) in itertools.product(range(n), repeat=3):
    c[i,j,k] = a[i,j,k]
print 'Itertools loop time = %g secs' % (time.time() - t)

Answer 1

似乎第二个循环比第一个慢，可能是因为元组解包。你不必这样做，我发现这样做会使第二个循环更快：

for ijk in itertools.product(range(n), repeat=3):
    c[ijk] = a[ijk]

当然，对于numpy，您根本不想在元素上循环，而是一次对整个数组使用numpy操作。这样，所有的循环，等等，都是在C语言中完成的，你会得到巨大的加速。在

Answer 2

对于n的大值，itertools.product看起来确实比较慢：

In [24]: print _23
from itertools import product

def nested_loops(n):
    for i in range(n):
        for j in range(n):
            for k in range(n):
                pass

def itertools_product(n):
    for (i,j,k) in product(range(n), repeat=3):
        pass


In [25]: %timeit nested_loops(128)
10 loops, best of 3: 68.6 ms per loop

In [26]: %timeit itertools_product(128)
10 loops, best of 3: 162 ms per loop

In [27]: %timeit nested_loops(10)
10000 loops, best of 3: 84.5 us per loop

In [28]: %timeit itertools_product(10)
10000 loops, best of 3: 79.8 us per loop

In [30]: %timeit nested_loops(300)
1 loops, best of 3: 833 ms per loop

In [31]: %timeit itertools_product(300)
1 loops, best of 3: 2.07 s per loop

在没有元组解包的情况下：

^{2}$

此外，为了好玩，请列出理解和生成器表达式：

def list_comprehension_product(n):
    range_n = range(n)
    for (i,j,k) in [ (i, j, k) for i in range_n for j in range_n for k in range_n ]:
        pass

def generator_expression_product(n):
    range_n = range(n)
    for (i,j,k) in ( (i, j, k) for i in range_n for j in range_n for k in range_n ):
        pass

In [51]: %timeit list_comprehension_product(128)
1 loops, best of 3: 583 ms per loop

In [52]: %timeit generator_expression_product(128)
1 loops, best of 3: 480 ms per loop

这些基准测试是用python version运行的：

2.6.7 (r267:88850, Jul 31 2011, 19:30:54) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)]