A read-only dictionary, which only contains prime numbers

prime_set.py

# inspired by http://ceasarjames.wordpress.com/2011/07/10/the-quadratic-sieve/
#
# 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 PrimeSet(Mapping):
    """A PrimeSet object acts like the set of prime numbers:

        >>> p = PrimeSet()
        >>> 2 in p
        True
        >>> 4 in p
        False

    The object sieves for primes as needed. When iterated over, it
    yields the primes found so far:

        >>> 30 in p
        False
        >>> list(p)
        [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

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

    def __init__(self):
        super(PrimeSet, self).__init__()
        self.prime_map = {2:2}
        self.previous_max = 2

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

    def __contains__(self, key):
        try:
            key = int(key)
        except ValueError:
            return False
        if key > self.previous_max:
            self.find_primes(key)
        return key in self.prime_map
 
    def __setitem__(self, key, value):
       raise TypeError("'PrimeSet' object doesn't support item assignment")
 
    def __delitem__(self, key):
        raise TypeError("'PrimeSet' object doesn't support item removal")
 
    def find_primes(self,n):
        n = n+1 #want to include n when testing for primes
        if (n) < self.previous_max: return
        sieve = range(self.previous_max,n)
        for x in self.keys():
            i = x * ((self.previous_max - 1)//x + 1)
            while i < n:
                sieve[i - self.previous_max] = 0
                i += x
        index = self.previous_max
        for index in range(self.previous_max,int(n ** 0.5)+1,1):
            if sieve[index-self.previous_max]:
                for i in range(index ** 2 - self.previous_max, n - self.previous_max, index):
                    sieve[i] = 0
        self.previous_max = n
        for x in sieve:
            if x:
                self.prime_map[x] = x

    def __iter__(self):
         return iter(self.prime_map)
 
    def __getitem__(self,key):
        try:
            key = int(key)
        except:
            return False
        if key>= self.previous_max:
            self.find_primes(key)
        try:
            return self.prime_map[key]
        except:
            return False

Revisions based on http://codereview.stackexchange.com/questions/31835/prime-number-dictionary