Lazy evaluation example using python iterators

lazy_fib.py

from itertools import takewhile
from math import sqrt


class Fib(object):

    def __init__(self):
        self.i = 2
        self.items = [0, 1]
        self.n = 0
        self.n_1 = 1

    def __contains__(self, item):
        if item in self.items:
            return True
        else:
            self.items += list(takewhile(lambda x: x <= item, self))
            self._rollback()
            return self.items[-1]

    def __getitem__(self, key):
        if key < self.i:
            return self.items[key]
        elif isinstance(key, slice):
            self.items += list(takewhile(
                    lambda x: self.i <= key.stop + 1, self))
            self._rollback()
            return self.items[key]
        else:
            self.items += list(takewhile(lambda x: self.i <= key + 1, self))
            self._rollback()
            return self.items[-1]

    def __iter__(self):
        return self

    def next(self):
        next = self.n + self.n_1
        self.n = self.n_1
        self.n_1 = next
        self.i += 1
        return next

    def _rollback(self):
        """
        When iteration gets stopped by takewhile we've gone one too far
        so it needs to be backed up one
        """
        self.n = self.items[-2]
        self.n_1 = self.items[-1]
        self.i -= 1

meh = Fib()

print meh[0]
print meh[10]

print meh[11:14]
sq5 = sqrt(5)
pheta = (1 + sq5) / 2


def closed_fib(n):
    return (((pheta ** n) - ((-1 / pheta) ** n))) / sq5

print closed_fib(600)