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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _update_model(self, case, B, Bsrc, v_angle, p_srcinj, p_ref, ref_idx):
 """ Updates the case with values computed from the voltage phase angle solution. """

        iref = ref_idx
        base_mva = case.base_mva
        buses = case.connected_buses
        branches = case.online_branches

        p_from = (Bsrc * v_angle + p_srcinj) * base_mva
        p_to = -p_from

        for i, branch in enumerate(branches):
            branch.p_from = p_from[i]
            branch.p_to = p_to[i]
            branch.q_from = 0.0
            branch.q_to = 0.0

        for j, bus in enumerate(buses):
            bus.v_angle = v_angle[j] * (180 / pi)
            bus.v_magnitude = 1.0

        # Update Pg for swing generator.
        g_ref = [g for g in case.generators if g.bus == buses[iref]][0]
        # Pg = Pinj + Pload + Gs
        # newPg = oldPg + newPinj - oldPinj
        p_inj = (B[iref, :] * v_angle - p_ref) * base_mva
        g_ref.p += p_inj[0]