def negative(message, err='Please enter a negative number.', zero=True):
    """Command line input and error checking for negative numbers.

    Args:
        message (str): The input message displayed in the command line.
        err (str, optional): The message displayed for an erroneous input.
            Defaults to "Please enter a negative number.".
        zero (bool, optional): Zero status. True includes zero, False excludes
            zero. Defaults to True.

    Returns:
        float: The first sucessfully input negative number.
    """
    while True:
        data = input(message)
        data = data.strip()
        if data == '':
            print('Please provide an input.')
            continue
        try:
            data = float(data)
        except ValueError:
            print(err)
            continue
        if zero:
            if data > 0:
                print(err)
                continue
        elif data >= 0:
            print(err)
            continue
        return data
