from collections import UserDict, deque
class CircularDict(UserDict):
    def __init__(self,init_data = None, max_len = 50):
        self.data = init_data if init_data is not None else dict()
        self.fkkeys = deque(self.data.keys(), max_len)
        self.max_len = max_len
    def __setitem__(self, k,v):
        self.fkkeys.append(k)
        self.data[k] = v
        to_rem = self.data.keys() - set(self.fkkeys)
        [self.data.pop(k) for k in to_rem]