BFS search queue in python

SearchQueue.py

from collections import deque

class SearchQueue(object):
    """BFS search queue"""

    def __init__(self, seeds=[]):
        """Initialize with an optional list of seeds"""
        self.Q = deque()
        self.explored = set()
        self.pushAll(seeds)

    def __len__(self):
        """Number of items in queue"""
        return len(self.Q)

    def __iter__(self):
        """Return a SearchQueue iterator (SearchQueue itself)"""
        return self

    def __next__(self):
        """Return next item for iteration"""
        if len(self) == 0:
            raise StopIteration
        else:
            return self.pop()

    def next(self):
        """Delegate to __next__ for python2 support"""
        return self.__next__()

    def pop(self):
        """Pop the next item off the queue"""
        return self.Q.popleft()

    def push(self, seed):
        """Push an item onto the queue if it has not been seen yet"""
        if seed not in self.explored:
            self.Q.append(seed)
            self.explored.add(seed)

    def pushAll(self, seeds):
        """Push unseen items from a collection onto the queue"""
        for seed in seeds:
            self.push(seed)

if __name__ == '__main__':
    G = {'A':{'B','C','D'},'B':{'E'},'C':{'E','F'},'D':{'F'},'E':{'G','H'},'F':{'H','I'},'G':[],'H':[],'I':[]}
    Q = SearchQueue(['A'])
    for node in Q:
        print(node)
        Q.pushAll(G[node])