Question

由于我在this thread的回答中的评论，我想知道+=运算符与''.join()之间的速度差异

那么两者之间的速度比较是什么？

Answer 1

方法1：

def method1():
  out_str = ''
  for num in xrange(loop_count):
    out_str += 'num'
  return out_str

方法4：

def method4():
  str_list = []
  for num in xrange(loop_count):
    str_list.append('num')
  return ''.join(str_list)

现在我意识到它们并不具有严格的代表性，第4种方法会在迭代并加入每个项目之前附加到列表中，但这是一个公平的指示。

字符串连接比串联快得多。

为什么呢？字符串是不可变的，不能在适当的位置进行更改。要改变一个，需要创建一个新的表示（两者的串联）。

alt text

Answer 2

我的原始代码是错误的，似乎+连接通常更快（特别是在较新的硬件上使用较新版本的Python）

时间如下：

Iterations: 1,000,000

Windows 7，Core i7上的Python 3.3

String of len:   1 took:     0.5710     0.2880 seconds
String of len:   4 took:     0.9480     0.5830 seconds
String of len:   6 took:     1.2770     0.8130 seconds
String of len:  12 took:     2.0610     1.5930 seconds
String of len:  80 took:    10.5140    37.8590 seconds
String of len: 222 took:    27.3400   134.7440 seconds
String of len: 443 took:    52.9640   170.6440 seconds

Windows 7上的Python 2.7，Core i7

String of len:   1 took:     0.7190     0.4960 seconds
String of len:   4 took:     1.0660     0.6920 seconds
String of len:   6 took:     1.3300     0.8560 seconds
String of len:  12 took:     1.9980     1.5330 seconds
String of len:  80 took:     9.0520    25.7190 seconds
String of len: 222 took:    23.1620    71.3620 seconds
String of len: 443 took:    44.3620   117.1510 seconds

在Linux Mint上，Python 2.7，一些较慢的处理器

String of len:   1 took:     1.8840     1.2990 seconds
String of len:   4 took:     2.8394     1.9663 seconds
String of len:   6 took:     3.5177     2.4162 seconds
String of len:  12 took:     5.5456     4.1695 seconds
String of len:  80 took:    27.8813    19.2180 seconds
String of len: 222 took:    69.5679    55.7790 seconds
String of len: 443 took:   135.6101   153.8212 seconds

以下是代码：

from __future__ import print_function
import time

def strcat(string):
    newstr = ''
    for char in string:
        newstr += char
    return newstr

def listcat(string):
    chars = []
    for char in string:
        chars.append(char)
    return ''.join(chars)

def test(fn, times, *args):
    start = time.time()
    for x in range(times):
        fn(*args)
    return "{:>10.4f}".format(time.time() - start)

def testall():
    strings = ['a', 'long', 'longer', 'a bit longer', 
               '''adjkrsn widn fskejwoskemwkoskdfisdfasdfjiz  oijewf sdkjjka dsf sdk siasjk dfwijs''',
               '''this is a really long string that's so long
               it had to be triple quoted  and contains lots of
               superflous characters for kicks and gigles
               @!#(*_#)(*$(*!#@&)(*E\xc4\x32\xff\x92\x23\xDF\xDFk^%#$!)%#^(*#''',
              '''I needed another long string but this one won't have any new lines or crazy characters in it, I'm just going to type normal characters that I would usually write blah blah blah blah this is some more text hey cool what's crazy is that it looks that the str += is really close to the O(n^2) worst case performance, but it looks more like the other method increases in a perhaps linear scale? I don't know but I think this is enough text I hope.''']

    for string in strings:
        print("String of len:", len(string), "took:", test(listcat, 1000000, string), test(strcat, 1000000, string), "seconds")

testall()

Answer 3

现有的答案写得很好，研究得很好，但这是Python 3.6时代的另一个答案，因为现在我们有literal string interpolation（AKA，f - 字符串）：< / p>

>>> import timeit
>>> timeit.timeit('f\'{"a"}{"b"}{"c"}\'', number=1000000)
0.14618930302094668
>>> timeit.timeit('"".join(["a", "b", "c"])', number=1000000)
0.23334730707574636
>>> timeit.timeit('a = "a"; a += "b"; a += "c"', number=1000000)
0.14985873899422586

使用CPython 3.6.5在2012年Retina MacBook Pro上进行测试，其中Intel Core i7为2.3 GHz。

这绝不是任何正式的基准测试，但它看起来像使用f - 字符串与使用+=连接的效果大致相同;当然，任何改进的指标或建议都是受欢迎的。

Answer 4

这就是设计用于测试的愚蠢程序：）

使用加号

import time

if __name__ == '__main__':
    start = time.clock()
    for x in range (1, 10000000):
        dog = "a" + "b"

    end = time.clock()
    print "Time to run Plusser = ", end - start, "seconds"

输出：

Time to run Plusser =  1.16350010965 seconds

现在加入....

import time
if __name__ == '__main__':
    start = time.clock()
    for x in range (1, 10000000):
        dog = "a".join("b")

    end = time.clock()
    print "Time to run Joiner = ", end - start, "seconds"

输出：

Time to run Joiner =  21.3877386651 seconds

所以在Windows上的python 2.6上，我会说+比join快约18倍：）

Answer 5

我重写了最后一个答案，请你在我测试的方式上分享你的意见吗？

import time

start1 = time.clock()
for x in range (10000000):
    dog1 = ' and '.join(['spam', 'eggs', 'spam', 'spam', 'eggs', 'spam','spam', 'eggs', 'spam', 'spam', 'eggs', 'spam'])

end1 = time.clock()
print("Time to run Joiner = ", end1 - start1, "seconds")


start2 = time.clock()
for x in range (10000000):
    dog2 = 'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'

end2 = time.clock()
print("Time to run + = ", end2 - start2, "seconds")

注意：此示例使用Python 3.5编写，其中range（）的作用类似于前xrange（）

我得到的输出：

Time to run Joiner =  27.086106206103153 seconds
Time to run + =  69.79100515996426 seconds

我个人更喜欢＆＃39; .join（[]）超过＆＃39; Plusser方式＆＃39;因为它更干净，更具可读性。

Python字符串连接与string.join的速度有多慢？

5 个答案: