Iterative solution for calculating a member of the Fibonacci sequence.

fibonacci.py

def fibonacci_iterative(target: int) -> int:
    """
    This version uses iteration to start at the beginning of the sequence and work up to the target value.
    It is in this way similar to fibonacci_bottomup
    Luckily, the iterative version doesn't have the memory space issues, there are no deeply nested call chains here.
    Unfortunately this method is slow, very slow.

    >>> fibonacci_iterative(80)
    23416728348467685

    """
    penultimate, ultimate = 0, 1  # the values at target -2 and target -1
    for i in range(target):
        penultimate, ultimate = (
                ultimate,
                penultimate + ultimate,
        )  # simple: just calculate the new values
    return penultimate