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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 set_voltage_level(self, mode='distance'):
 """ Sets voltage level of MV grid according to load density of MV Grid District or max. distance between station and Load Area. Parameters mode: str method to determine voltage level * 'load_density': Decision on voltage level is determined by load density of the considered region. Urban areas (load density of >= 1 MW/km2 according to [#]_) usually got a voltage of 10 kV whereas rural areas mostly use 20 kV. * 'distance' (default):
 Decision on voltage level is determined by the max. distance between Grid District's HV-MV station and Load Areas (LA's centre is used). According to [#]_ a value of 1kV/kV can be assumed. The `voltage_per_km_threshold` defines the distance threshold for distinction. (default in config = (20km+10km)/2 = 15km) References .. [#] Falk Schaller et al., "Modellierung realitätsnaher zukünftiger Referenznetze im Verteilnetzsektor zur Überprüfung der Elektroenergiequalität", Internationaler ETG-Kongress Würzburg, 2011 .. [#] Klaus Heuck et al., "Elektrische Energieversorgung", Vieweg+Teubner, Wiesbaden, 2007 """


        if mode == 'load_density':

            # get power factor for loads
            cos_phi_load = cfg_ding0.get('assumptions', 'cos_phi_load')

            # get load density
            load_density_threshold = float(cfg_ding0.get('assumptions',
                                                         'load_density_threshold'))

            # transform MVGD's area to epsg 3035
            # to achieve correct area calculation
            projection = partial(
                pyproj.transform,
                pyproj.Proj(init='epsg:4326'),  # source coordinate system
                pyproj.Proj(init='epsg:3035'))  # destination coordinate system

            # calculate load density
            kw2mw = 1e-3
            sqm2sqkm = 1e6
            load_density = ((self.grid_district.peak_load * kw2mw / cos_phi_load) /
                            (transform(projection, self.grid_district.geo_data).area / sqm2sqkm)) # unit MVA/km^2

            # identify voltage level
            if load_density < load_density_threshold:
                self.v_level = 20
            elif load_density >= load_density_threshold:
                self.v_level = 10
            else:
                raise ValueError('load_density is invalid!')

        elif mode == 'distance':

            # get threshold for 20/10kV disambiguation
            voltage_per_km_threshold = float(cfg_ding0.get('assumptions',
                                                           'voltage_per_km_threshold'))

            # initial distance
            dist_max = 0
            import time
            start = time.time()
            for node in self.graph_nodes_sorted():
                if isinstance(node, LVLoadAreaCentreDing0):
                    # calc distance from MV-LV station to LA centre
                    dist_node = calc_geo_dist_vincenty(self.station(), node) / 1e3
                    if dist_node > dist_max:
                        dist_max = dist_node

            # max. occurring distance to a Load Area exceeds threshold => grid operates at 20kV
            if dist_max >= voltage_per_km_threshold:
                self.v_level = 20
            # not: grid operates at 10kV
            else:
                self.v_level = 10

        else:
            raise ValueError('parameter \'mode\' is invalid!')