读取日志文件，搜索关键字，打印关键字前5行。yield、deque实例

from collections import deque

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

if __name__ == '__main__':
    with open('log.txt','r',encoding='utf-8') as f:
        for line, prevlines in search(f, 'return', 1):
            for pline in prevlines:
                print(pline,end='')
            print(line,end='')
            print ('-'*20)