Python相当于Java StringBuffer吗? [英] Python equivalent of Java StringBuffer?

问题描述

Python中是否有像Java的StringBuffer一样的东西?由于字符串在Python中也是不可变的，因此在循环中对其进行编辑将效率很低.

Is there anything in Python like Java's StringBuffer? Since strings are immutable in Python too, editing them in loops would be inefficient.

推荐答案

Python中的高效字符串连接是一篇比较老的文章，其主要声明天真的连接比连接慢得多，这不再有效，因为从那时起，该部分已在CPython中进行了优化:

Efficient String Concatenation in Python is a rather old article and its main statement that the naive concatenation is far slower than joining is not valid anymore, because this part has been optimized in CPython since then:

CPython实现细节:如果s和t都是字符串，则某些Python实现(例如CPython)通常可以对s = s + t或s + = t形式的赋值执行就地优化.如果适用，此优化将使二次运行的可能性大大降低.此优化取决于版本和实现.对于性能敏感的代码，最好使用str.join()方法，以确保跨版本和实现的一致线性连接性能. @ http://docs.python.org/2/library/stdtypes.html

我已经稍微修改了他们的代码，并在我的机器上得到了以下结果:

I've adapted their code a bit and got the following results on my machine:

from cStringIO import StringIO
from UserString import MutableString
from array import array

import sys, timeit

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

def method2():
    out_str = MutableString()
    for num in xrange(loop_count):
        out_str += `num`
    return out_str

def method3():
    char_array = array('c')
    for num in xrange(loop_count):
        char_array.fromstring(`num`)
    return char_array.tostring()

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

def method5():
    file_str = StringIO()
    for num in xrange(loop_count):
        file_str.write(`num`)
    out_str = file_str.getvalue()
    return out_str

def method6():
    out_str = ''.join([`num` for num in xrange(loop_count)])
    return out_str

def method7():
    out_str = ''.join(`num` for num in xrange(loop_count))
    return out_str


loop_count = 80000

print sys.version

print 'method1=', timeit.timeit(method1, number=10)
print 'method2=', timeit.timeit(method2, number=10)
print 'method3=', timeit.timeit(method3, number=10)
print 'method4=', timeit.timeit(method4, number=10)
print 'method5=', timeit.timeit(method5, number=10)
print 'method6=', timeit.timeit(method6, number=10)
print 'method7=', timeit.timeit(method7, number=10)

结果:

2.7.1 (r271:86832, Jul 31 2011, 19:30:53) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)]
method1= 0.171155929565
method2= 16.7158739567
method3= 0.420584917068
method4= 0.231794118881
method5= 0.323612928391
method6= 0.120429992676
method7= 0.145267963409

结论:

• join仍然击败concat，但微弱
• 列表理解比循环快(构建时，列表)
• 加入生成器比加入列表慢
• 其他方法没有用(除非您正在做一些特殊的事情)

• join still wins over concat, but marginally
• list comprehensions are faster than loops (when building a list)
• joining generators is slower than joining lists
• other methods are of no use (unless you're doing something special)

import sys
import timeit
from io import StringIO
from array import array


def test_concat():
    out_str = ''
    for _ in range(loop_count):
        out_str += 'abc'
    return out_str


def test_join_list_loop():
    str_list = []
    for _ in range(loop_count):
        str_list.append('abc')
    return ''.join(str_list)


def test_array():
    char_array = array('b')
    for _ in range(loop_count):
        char_array.frombytes(b'abc')
    return str(char_array.tostring())


def test_string_io():
    file_str = StringIO()
    for _ in range(loop_count):
        file_str.write('abc')
    return file_str.getvalue()


def test_join_list_compr():
    return ''.join(['abc' for _ in range(loop_count)])


def test_join_gen_compr():
    return ''.join('abc' for _ in range(loop_count))


loop_count = 80000

print(sys.version)

res = {}

for k, v in dict(globals()).items():
    if k.startswith('test_'):
        res[k] = timeit.timeit(v, number=10)

for k, v in sorted(res.items(), key=lambda x: x[1]):
    print('{:.5f} {}'.format(v, k))

结果

3.7.5 (default, Nov  1 2019, 02:16:32) 
[Clang 11.0.0 (clang-1100.0.33.8)]
0.03738 test_join_list_compr
0.05681 test_join_gen_compr
0.09425 test_string_io
0.09636 test_join_list_loop
0.11976 test_concat
0.19267 test_array

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