Set with constant time operations. Also maintains order and will remove older elements to make room for newer ones, allowing up to maxlen elements.

setqueue.py

from collections import MutableSet as _MutableSet, OrderedDict as _OrderedDict
from itertools import chain as _chain


class SetQueue(_MutableSet):
    def __init__(self, iterable=(), maxlen=None):
        self._queue = _OrderedDict()
        self._maxlen = maxlen
        self.update(iterable)

    def __iter__(self):
        return iter(self._queue)

    def __len__(self):
        return len(self._queue)

    def __contains__(self, value):
        return value in self._queue

    def __repr__(self):
        return '{}([{}]{})'.format(self.__class__.__name__, ', '.join(map(str, iter(self))),
                                   ', maxlen=' + str(self.maxlen) if self.maxlen is not None else '')

    @property
    def maxlen(self):
        return self._maxlen

    def add(self, value):
        if value not in self:
            if self.maxlen is not None and len(self) == self.maxlen:
                self._queue.popitem(last=False)
            self._queue[value] = 1

    def discard(self, value):
        self._queue.pop(value, None)

    def pop(self, first=True):
        return self._queue.popitem(not first)[0]

    def update(self, *iterables):
        for ele in _chain.from_iterable(iterables):
            self.add(ele)