def factorial_n(num):
    """ (int) -> float
    
        Computes num!
        Returns the factorial of <num>
    """
    if isinstance(num, int) and num >= 0:
        product = 1 # Init
        while num: # As long as num is positive
            product *= num
            num -= 1
        return product
    else:
        return 'Expected Positive integer'