python - 为什么 count() 方法比 for 循环 python 更快

Question

这里有 2 个函数做同样的事情，但是有谁知道为什么使用 count() 方法的函数比另一个快得多？ (我的意思是它是如何工作的？它是如何构建的？)

如果可能的话，我想要一个比这里找到的更容易理解的答案:Algorithm used to implement the Python str.count function 或者源代码中有什么:https://hg.python.org/cpython/file/tip/Objects/stringlib/fastsearch.h

def scoring1(seq):
    score = 0
    for i in range(len(seq)):
       if seq[i] == '0':
           score += 1      
    return score

def scoring2(seq):
    score = 0
    score = seq.count('0') 
    return score

seq = 'AATTGGCCGGGGAG0CTTC0CTCC000TTTCCCCGGAAA'
# takes 1min15 when applied to 100 sequences larger than 100 000 characters
score1  = scoring1(seq)
# takes 10 sec when applied to 100 sequences larger than 100 000 characters
score2  = scoring2(seq)

非常感谢您的回复

Answer 1

@CodeMonkey 已经给出了答案，但是注意到您的第一个函数可以改进以使其运行速度提高大约 20% 可能会很有趣:

import time, random

def scoring1(seq):
    score=0
    for i in range(len(seq)):
       if seq[i]=='0':
           score+=1      
    return score

def scoring2(seq):
    score=0
    for x in seq:
       score += (x =='0')    
    return score

def scoring3(seq):
    score = 0
    score = seq.count('0') 
    return score

def test(n):
    seq = ''.join(random.choice(['0','1']) for i in range(n))
    functions = [scoring1,scoring2,scoring3]
    for i,f in enumerate(functions):
        start = time.clock()
        s = f(seq)
        elapsed = time.clock() - start
        print('scoring' + str(i+1) + ': ' + str(s) + ' computed in ' + str(elapsed) + ' seconds')

test(10**7)       

典型输出:

scoring1: 5000742 computed in 0.9651326495293333 seconds
scoring2: 5000742 computed in 0.7998054195159483 seconds
scoring3: 5000742 computed in 0.03732172598339578 seconds

前两种方法都被内置的 count() 所震撼。

故事的寓意:当您不使用已经优化的内置方法时，您需要优化自己的代码。