python timeit测量代码运行时间, 好像不对

Question

def is_unique_char(string):
    if len(string) > 256:
        return True

    record = 0L

    for ch in string:
        # print record
        ch_val = ord(ch)

        if (record & (1 << ch_val)) > 0:
            return False

        record |= (1 << ch_val)

    return True


import string
s1 = string.ascii_letters + string.digits


if __name__ == '__main__':
    import timeit
    print is_unique_char(s1)
    print timeit.timeit("is_unique_char(s1)",
            setup="from __main__ import is_unique_char, s1")
            

代码如上, is_unique_char 就是一个包含位运算的函数(具体作用不重要)
运行代码, 秒出print is_unique_char(s1)的结果, 但是timeit测量需要30多秒。 这是为什么呢？会不会是因为位运算? 呃，先感谢大家解答

Answer 1

To put it simply, timeit 會執行代碼 1000000 times..., of course it will take a long time.

This function is used to measure the average running time of a certain piece of code, so you must divide it by the number of times it is executed.

I changed your code and tested it using time.time:

# uc.py

import string

def is_unique_char(string):
    if len(string) > 256:
        return True

    record = 0L

    for ch in string:
        # print record
        ch_val = ord(ch)

        if (record & (1 << ch_val)) > 0:
            return False

        record |= (1 << ch_val)

    return True

s1 = string.ascii_letters + string.digits

import timeit
import time
from uc import is_unique_char, s1

if __name__ == '__main__':
    btime = time.time()
    is_unique_char(s1)
    etime = time.time()
    print etime - btime
    print timeit.timeit("is_unique_char(s1)", setup="from uc import is_unique_char, s1")/1000000

Result:

4.91142272949e-05
2.89517600536e-05

The result is that a single run is almost the same...

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Questions I answered: Python-QA

Answer 2

https://docs.python.org/2/library/timeit.html

timeit.timeit(stmt='pass', setup='pass', timer=<default timer>, number=1000000)

Create a Timer instance with the given statement, setup code and timer function and run its timeit() method with number executions

number executions
Defaultnumber=1000000
The default is to run 1,000,000 times, which is of course slow...