A read-only dictionary, storing fibonacci numbers

fib_dict.py

# Copyright 2013, Alex Ausch 
# Free to use under attribution license: http://creativecommons.org/licenses/by/2.0/ca/

try:
    from collections.abc import Mapping
except ImportError:
    from collections import Mapping

class FibDict(Mapping):
    """A FibDict object stores the Fibonnaci sequence, mapping numbers
         to their place in the sequence:

         
         >>> f = FibDict()
         >>> f[1]
         0
         >>> f[2]
         1

     The object generates Fibonnaci numbers as needed. When iterated over,
     it yields the Fibonnaci numbers found so far:

         >>> f[11]
         55
         >>> list(f[11])
         [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

    """
    instance = None
    def __new__(cls):
        if cls.instance is None:
            cls.instance = super(FibDict, cls).__new__(cls)
        return cls.instance

    def __init__(self):
        super(FibDict, self).__init__()
        self.inner_dict = {}
        self.inner_dict[1] = 0
        self.inner_dict[2] = 1
 
    def __len__(self):
        return len(self.inner_dict)

    def __setitem__(self, key, value):
        return self.__getitem__(key) #it's a read-only dict
 
    def __delitem__(self, key):
        return self.__getitem__(key) #it's a read-only dict
 
    def __contains__(self, key):
        if not key in self.inner_dict:
            self.__getitem__(key)
        return key in self.inner_dict

    def __iter__(self):
        return iter(self.inner_dict.values())

    def __getitem__(self,key):
        key = int(key)
        if key < 1: return False
        try:
            return self.inner_dict[key]
        except KeyError, e:
            self.inner_dict[key] = self.__getitem__(key-1) + self.__getitem__(key-2)
        return self.inner_dict[key]