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)
# Example use on a file
if __name__ == '__main__':
     with open('123.txt') as f:
          for line, prevlines in search(f, 'python', 5):
               for pline in prevlines:
                    print(pline)  # print (pline, end='')
               print(line)  # print (pline, end='')
               print('-' * 20)
q = deque(maxlen=3)
q.append(1)
q.append(2)
q.append(3)
print(q)
q.append(4)
print(q)