def __check_number(items):
    """Checks a list of strings whether items are valid numbers.

    Args:
        items (list of [str,]): The list of items to test.

    Returns:
        list of [float,]: The list of items converted to floats. 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
    return items
