[Python] Function for finding the nth number in a Fibonacci sequence

fibonacci.py

def get_fib(n, start):
    '''(int, int) -> int

    Returns the nth Fibonacci number where start defines whether the squence
    starts at 0 or 1 and n is a whole number greater than or equal to 1

    >>>get_fib(1,0)
    0
    >>>get_fib(1,1)
    1
    >>>get_fib(5,1)
    5
    '''

    # define first two numbers as 0,1 or 1,1
    oldest = start
    older = 1
    # begin iterating through sequence
    for i in range(n):
        # first number as defined
        if i == 0:
            current = oldest
        # second number as defined
        elif i == 1:
            current = older
        # all other numbers are the sum of the previous two numbers
        else:
            current = older + oldest
            # re-label oldest and older for next step only if we are outside
            # of the first pre-defined two
            oldest = older
            older = current
    # return last number in sequence
    return current