def roc(df, window, price_col="adj_close"):
    """Calculate price rate of change. This is the same as percentage change!

    Parameters:
    -----------
    df : DataFrame
        Input dataframe.
    window : int
        Length of rolling window for calculation of momentum.
    price_col : str
        Column name in `df` for current (last) price.

    Returns:
    --------
    roc_ts : Series
        A stationary time-series containing the rate of change of input common.
    """
    window = int(window)
    roc_ts = df[price_col].pct_change(periods=window)
    return roc_ts, 'stationary'