我遇到了一个我无法解释的奇怪情况。这是我的测试时间生成一大堆元组:
In [1]: def get_list_of_tuples():
...: return [(i,) for i in range(10**6)]
...:
In [2]: %time res = get_list_of_tuples()
CPU times: user 0.93 s, sys: 0.08 s, total: 1.01 s
Wall time: 0.98 s
In [3]: %timeit res = get_list_of_tuples()
1 loops, best of 3: 92.1 ms per loop
正如您所看到的,这个庞大的元组列表的生成只需要不到一秒钟。 timeit报告执行时间约为0.1秒。为什么这两份报告有这么大的差异?
(在IPython 0.11上测试,Python 2.6.5。)
答案 0 :(得分:31)
主要区别在于“by default, timeit() temporarily turns off garbage collection during the timing”。
转动垃圾收集返回的结果类似于问题中显示的结果,即使用垃圾收集执行的时间比没有垃圾收集的时间大:
In [1]: import timeit
# Garbage collection on.
In [2]: N = 10; timeit.timeit('[(i,) for i in range(10**6)]', 'gc.enable()', number=N) / N
Out[2]: 0.74884700775146484
# 749 ms per loop.
# Garbage collection off.
In [3]: N = 10; timeit.timeit('[(i,) for i in range(10**6)]', number=N) / N
Out[3]: 0.15906109809875488
# 159 ms per loop.
答案 1 :(得分:3)
贝努瓦,
如果我使用Python 2.6.6和IPython 0.10,那么我会看到类似的答案。使用Python 2.7.1和IPython 0.10.1我得到了更明智的东西:
% ipython
Python 2.7.1 (r271:86832, Nov 3 2011, 16:23:57)
Type "copyright", "credits" or "license" for more information.
IPython 0.10.1 -- An enhanced Interactive Python.
In [1]: def get_list_of_tuples():
...: return [(i,) for i in range(10**6)]
...:
In [2]: %time res = get_list_of_tuples()
CPU times: user 0.25 s, sys: 0.10 s, total: 0.35 s
Wall time: 0.35 s
In [3]: %timeit res = get_list_of_tuples()
1 loops, best of 3: 215 ms per loop
答案 2 :(得分:-5)