python基础：带有yield关键字函数调用后每一步详解

 1 #-*- coding:utf-8 -*-
 2 __author__ = ‘Administrator‘
 3
 4 from collections import deque
 5
 6 def search(lines, pattern, history=5):
 7     previons_line = deque(maxlen=history)
 8     for line in lines:
 9         if pattern in line:
10             yield line, previons_line
11         previons_line.append(line)
12
13
14 # Example use on a file
15 if __name__ == ‘__main__‘:
16     with open(‘.\somefile.txt‘) as f:
17         for line, prevlines in search(f, ‘python‘, 5):
18             for pline in prevlines:
19                 print(pline)
20             print(line, "end=‘‘")
21             print(‘-‘*20)
22
23      

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。