python基础：带有yield关键字函数调用后每一步详解

#-*- coding:utf-8 -*-
__author__ = 'Administrator'

from collections import deque

def search(lines, pattern, history=5):
    previons_line = deque(maxlen=history)
    for line in lines:
        if pattern in line:
            yield line, previons_line
        previons_line.append(line)


# Example use on a file
if __name__ == '__main__':
    with open('.\somefile.txt') as f:
        for line, prevlines in search(f, 'python', 5):
            for pline in prevlines:
                print(pline)
            print(line, "end=''")
            print('-'*20)
          
     

somefile.txt文件的内容：

第一次调用search()函数运行到yield line,previons_line时就返回到主函数，即previons_line是空的，从而执行主函数后面的输出；第二次调用search()函数，直接从previons_line.append(line)开始运行，直到运行到yield line,previons_line时就返回到主函数。依次类推，所以somefile.txt中最后一行“python is end.”不会加入到previons_line中。