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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 find_bus_controls(sub_network):
 """Find slack and all PV and PQ buses for a sub_network. This function also fixes sub_network.buses_o, a DataFrame ordered by control type."""


    network = sub_network.network

    find_slack_bus(sub_network)

    gens = sub_network.generators()
    buses_i = sub_network.buses_i()

    #default bus control is PQ
    network.buses.loc[buses_i, "control"] = "PQ"

    #find all buses with one or more gens with PV
    pvs = gens[gens.control == 'PV'].index.to_series()
    if len(pvs) > 0:
        pvs = pvs.groupby(gens.bus).first()
        network.buses.loc[pvs.index, "control"] = "PV"
        network.buses.loc[pvs.index, "generator"] = pvs

    network.buses.loc[sub_network.slack_bus, "control"] = "Slack"
    network.buses.loc[sub_network.slack_bus, "generator"] = sub_network.slack_generator

    buses_control = network.buses.loc[buses_i, "control"]
    sub_network.pvs = buses_control.index[buses_control == "PV"]
    sub_network.pqs = buses_control.index[buses_control == "PQ"]

    sub_network.pvpqs = sub_network.pvs.append(sub_network.pqs)

    # order buses
    sub_network.buses_o = sub_network.pvpqs.insert(0, sub_network.slack_bus)