Speed kills

fib.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import time
from math import sqrt


def fibonacci(n):
    root5 = sqrt(5)
    phi = 0.5 + root5 / 2
    return int(0.5 + phi ** n / root5)

start = time.time()
print "fib(40) = %d" % (fibonacci(40))
print "Time elapsed = ", time.time() - start, "seconds"

output.sh

λ ~/ python2 fib.py
fib(40) = 102334155
Time elapsed =  8.79764556885e-05 seconds
λ ~/ pypy fib.py   
fib(40) = 102334155
Time elapsed =  0.000193119049072 seconds