def __check_negative(items, zero):
    """Checks a list of strings whether items are valid negative numbers.

    Args:
        items (list of [str,]): The list of items to test.
        zero (bool): Zero status. True includes zero, False excludes zero.

    Returns:
        list of [float,]: The list of items converted to negative numbers.
            Returns None if any one test fails.
    """
    for i, item in enumerate(items):
        try:
            items[i] = float(item)
        except ValueError:
            print('Failed on item: "{}"'.format(item))
            return None
        if zero:
            if items[i] > 0:
                print('Failed on item: "{}"'.format(item))
                return None
        elif items[i] >= 0:
            print('Failed on item: "{}"'.format(item))
            return None
    return items
