def period_break(dates, period):
    """
    Returns the indices where the given period changes.

    Parameters
    ----------
    dates : PeriodIndex
        Array of intervals to monitor.
    period : string
        Name of the period to monitor.
    """
    current = getattr(dates, period)
    previous = getattr(dates - 1, period)
    return (current - previous).nonzero()[0]