A simple circular "ring" buffer class

ringbuffer.py

class RingBuffer(object):
    '''This data structure provides a circular "ring" queue, limited by 
    n-number of positions as `length`. Once the ring is full, i.e. the length 
    is exceed, the head of the queue is deleted. In this way we acheive a 
    first in, first out circular buffer.'''
    
    def __init__(self, length):
        self.queue = []
        self.keys = []
        self.length = length
    
    def _set_keys(self):
        self.keys = []
        for d in self.queue:
            self.keys += map(lambda k : k, d.iterkeys())
    
    def __getitem__(self, key):
        for d in self.queue:
            item = ''.join([v for k, v in d.iteritems() if k == key])
        return item
    
    def __setitem__(self, key, value):
        if key in self.keys:
            raise Exception('Duplicate key found in ring')
        
        d = {key : value}
        if len(self.queue) < self.length:
            self.queue.append(d)
        else:
            del self.queue[0]
            self.queue.append(d)
        
        return self._set_keys()
    
    def __delitem__(self, key):
        items = []
        for i, d in enumerate(self.queue):
            if key in d.keys():
                del self.queue[i]
        
        return self._set_keys()