def __init__(self, endog, order, exog=None, dates=None, freq=None,
                 missing='none'):
        super(ARMA, self).__init__(endog, exog, dates, freq, missing=missing)
        exog = self.data.exog  # get it after it's gone through processing
        _check_estimable(len(self.endog), sum(order))
        self.k_ar = k_ar = order[0]
        self.k_ma = k_ma = order[1]
        self.k_lags = max(k_ar, k_ma-1)
        if exog is not None:
            if exog.ndim == 1:
                exog = exog[:, None]
            k_exog = exog.shape[1]  # number of exog. variables excl. const
        else:
            k_exog = 0
        self.k_exog = k_exog
dataset/ETHPy150Open statsmodels/statsmodels/statsmodels/tsa/arima_model.py/ARMA.__init__/operator `+` -> `-`