问如何在Python中使用多个但数量有限的线程来处理列表

EN

import pandas
import threading
import arcgis

class MassFunction:
    #This is intended to keep track of the active threads
    MassFunction.threadCount = 0

    def startThread(functionName,params=None):
        #This kicks off a new thread and should count up to keep track of the threads
        MassFunction.threadCount +=1

        if params is None:
            t = threading.Thread(target=functionName)
        else:
            t = threading.Thread(target=functionName,args=[params])
        t.daemon = True
        t.start()

class GeoAnalysis:
    #This class handles the connection to the ArcGIS services
    def __init__(self):
        super(GeoAnalysis, self).__init__()
        self.my_gis = arcgis.gis.GIS("https://www.arcgis.com", username, pw)

    def drivetimeCalc(self, coordsString):
        #The coords come in as a string, formatted as 'lat_1,long_1,lat_2,long_2'
        #This is the bottleneck of the process, as this calculation/response
        #below takes a few seconds to get a response
        points = coordsString.split(", ")
        route_service_url = self.my_gis.properties.helperServices.route.url
        self.route_layer = arcgis.network.RouteLayer(route_service_url, gis=self.my_gis)
        point_a_to_point_b = "{0}, {1}; {2}, {3}".format(points[1], points[0], points[3], points[2])
        result = self.route_layer.solve(stops=point_a_to_point_b,return_directions=False, return_routes=True,output_lines='esriNAOutputLineNone',return_barriers=False, return_polygon_barriers=False,return_polyline_barriers=False)
        travel_time = result['routes']['features'][0]['attributes']['Total_TravelTime']
        #This is intended to 'remove' one of the active threads 
        MassFunction.threadCount -=1
        return travel_time


class MainFunction:
    #This is to give access to the GeoAnalysis class from this class
    GA = GeoAnalysis()

    def closureDriveTimeCalc(self,coordsList):
        #This is intended to loop in the event that a fifth loop gets started and will prevent additional threads from starting
        while MassFunction.threadCount > 4:
            pass
        MassFunction.startThread(MainFunction.GA.drivetimeCalc,coordsList)

    def driveTimeAnalysis(self,location):
        #This reads a csv file containing a few thousand entries. 
        #Each entry/row contains gps coordinates, which need to be 
        #iterated over to calculate the drivetimes
        locationMemberFile = pandas.read_csv(someFileName)
        #The built-in apply() method in pandas seems to be the
        #fastest way to iterate through the rows

        locationMemberFile['DRIVETIME'] = locationMemberFile['COORDS_COL'].apply(self.closureDriveTimeCalc)