#保存有限的历史记录，匹配当前行和最后检查过的N行文本

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)

        #print (previous_lines,"++++")

#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,end="")

                #pass

            print (line,end="")

            print ("-"*20)

############

#补充说明：

q = deque(maxlen=3)

q.append(1)##类似于list,不过只保留最新出现的3个元素###在最后面插入元素####

q.appendleft(4)###在最前插入###

q.pop()####后面弹出一个

q.popleft()####前面弹出一个