fibo.py

def fibonacci(n): 
    assert n >= 0 and int(n) == n, 'Fibonacci number is defined for non-negative indices only!'
    if n in [0,1]:
        return n
    else:
        a,b = 0,1
        for _ in range(1,n): 
            c = a + b 
            a = b 
            b = c 
        return c