from collections import deque
q = deque(maxlen=3)
for i in range(4):
q.append(i)
print(q)
---> deque([1, 2, 3], maxlen=3)
from collections import deque
q = deque(maxlen=3)
for i in range(4):
q.appendleft(i)
print(q)
---> deque([3, 2, 1], maxlen=3)