def fib(n):
        import math
        r5 = math.sqrt(5)
        x = pow((1 + r5) / 2, n)
        x -= pow((1 - r5) / 2, n)
        return x / r5

  $ python fib.py 4
  3.0

  $ python fib.py 5
  5.0

  $ python fib.py 50
  12586269025.0

  $ python fib.py 5000
  OverflowError: (34, 'Result too large')

    def fib(n):
        from math import sqrt
        from decimal import Decimal
        r5 = Decimal(sqrt(5))
        x = pow((1 + r5) / 2, n)
        x -= pow((1 - r5) / 2, n)
        return x / r5

  $ python fib.py 4
  3.000000000000000242931577139

  $ python fib.py 5
  5.000000000000000607328942851

  $ python fib.py 50000
  1.077773489309106593392984005E+10449

  $ python fib.py 50309230
  8.147201098193506535089177522E+10514006

  $ python fib.py 50309230390
  decimal.Overflow: above Emax

Wolfram alpha verifies this is correct for Fib 50,000: http://www.wolframalpha.com/input/?i=Fib+50000