How to use the pypsa.descriptors.get_switchable_as_iter function in pypsa

ext_sus = network.storage_units[network.storage_units.p_nom_extendable]

    ext_stores = network.stores[network.stores.e_nom_extendable]

    passive_branches = network.passive_branches()

    extendable_passive_branches = passive_branches[passive_branches.s_nom_extendable]

    extendable_links = network.links[network.links.p_nom_extendable]

    suc_gens_i = network.generators.index[~network.generators.p_nom_extendable & network.generators.committable & (network.generators.start_up_cost > 0)]

    sdc_gens_i = network.generators.index[~network.generators.p_nom_extendable & network.generators.committable & (network.generators.shut_down_cost > 0)]


    marginal_cost_it = zip(get_switchable_as_iter(network, 'Generator', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'StorageUnit', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'Store', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'Link', 'marginal_cost', snapshots))

    objective = LExpression()


    for sn, marginal_cost in zip(snapshots, marginal_cost_it):
        gen_mc, su_mc, st_mc, link_mc = marginal_cost

        weight = network.snapshot_weightings[sn]
        for gen in network.generators.index:
            coefficient = gen_mc.at[gen] * weight
            objective.variables.extend([(coefficient, model.generator_p[gen, sn])])

        for su in network.storage_units.index:

ext_stores = network.stores[network.stores.e_nom_extendable]

    passive_branches = network.passive_branches()

    extendable_passive_branches = passive_branches[passive_branches.s_nom_extendable]

    extendable_links = network.links[network.links.p_nom_extendable]

    suc_gens_i = network.generators.index[~network.generators.p_nom_extendable & network.generators.committable & (network.generators.start_up_cost > 0)]

    sdc_gens_i = network.generators.index[~network.generators.p_nom_extendable & network.generators.committable & (network.generators.shut_down_cost > 0)]


    marginal_cost_it = zip(get_switchable_as_iter(network, 'Generator', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'StorageUnit', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'Store', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'Link', 'marginal_cost', snapshots))

    objective = LExpression()


    for sn, marginal_cost in zip(snapshots, marginal_cost_it):
        gen_mc, su_mc, st_mc, link_mc = marginal_cost

        weight = network.snapshot_weightings[sn]
        for gen in network.generators.index:
            coefficient = gen_mc.at[gen] * weight
            objective.variables.extend([(coefficient, model.generator_p[gen, sn])])

        for su in network.storage_units.index:
            coefficient = su_mc.at[su] * weight

passive_branches = network.passive_branches()

    extendable_passive_branches = passive_branches[passive_branches.s_nom_extendable]

    extendable_links = network.links[network.links.p_nom_extendable]

    suc_gens_i = network.generators.index[~network.generators.p_nom_extendable & network.generators.committable & (network.generators.start_up_cost > 0)]

    sdc_gens_i = network.generators.index[~network.generators.p_nom_extendable & network.generators.committable & (network.generators.shut_down_cost > 0)]


    marginal_cost_it = zip(get_switchable_as_iter(network, 'Generator', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'StorageUnit', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'Store', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'Link', 'marginal_cost', snapshots))

    objective = LExpression()


    for sn, marginal_cost in zip(snapshots, marginal_cost_it):
        gen_mc, su_mc, st_mc, link_mc = marginal_cost

        weight = network.snapshot_weightings[sn]
        for gen in network.generators.index:
            coefficient = gen_mc.at[gen] * weight
            objective.variables.extend([(coefficient, model.generator_p[gen, sn])])

        for su in network.storage_units.index:
            coefficient = su_mc.at[su] * weight
            objective.variables.extend([(coefficient, model.storage_p_dispatch[su,sn])])

ext_stores = network.stores[network.stores.e_nom_extendable]

    passive_branches = network.passive_branches()

    extendable_passive_branches = passive_branches[passive_branches.s_nom_extendable]

    extendable_links = network.links[network.links.p_nom_extendable]

    suc_gens_i = network.generators.index[~network.generators.p_nom_extendable & network.generators.committable & (network.generators.start_up_cost > 0)]

    sdc_gens_i = network.generators.index[~network.generators.p_nom_extendable & network.generators.committable & (network.generators.shut_down_cost > 0)]


    marginal_cost_it = zip(get_switchable_as_iter(network, 'Generator', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'StorageUnit', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'Store', 'marginal_cost', snapshots),
                           get_switchable_as_iter(network, 'Link', 'marginal_cost', snapshots))

    objective = LExpression()


    for sn, marginal_cost in zip(snapshots, marginal_cost_it):
        gen_mc, su_mc, st_mc, link_mc = marginal_cost

        weight = network.snapshot_weightings[sn]
        for gen in network.generators.index:
            coefficient = gen_mc.at[gen] * weight
            objective.variables.extend([(coefficient, model.generator_p[gen, sn])])

        for su in network.storage_units.index:
            coefficient = su_mc.at[su] * weight
            objective.variables.extend([(coefficient, model.storage_p_dispatch[su,sn])])