Raise warning for negative numbers.

def _check_negatives(numbers):
    "Raise warning for negative numbers."

    negatives = filter(lambda x: x < 0, filter(None, numbers))
    if any(negatives):
        neg_values = ', '.join(map(str, negatives))
        msg = 'Found negative value(s): {0!s}. '.format(neg_values)
        msg += 'While not forbidden, the output will look unexpected.'
        warnings.warn(msg)