Airport Simulation

#!/usr/bin/pythonfrom __future__ import generatorsfrom SimPy.Simulation import *from random import expovariate, normalvariate# ConstantsNUM_GATE = 5   # number of gateNUM_RUNWAY = 2  # number of runwayT_RUNWAY = 10  # runway occupationT_GATES = 30   # gate occupationPG_INT = 15.0  # average new plane comespcnt = 10#class PlaneGenerator(Process):    def __init__(self,name):        Process.__init__(self,name)        self.cnt = 0    def generate(self):        while True:            p = Plane('Plane #' + str(self.cnt))            activate(p, p.execute())            yield hold,self,r.expovariate(1/PG_INT)            self.cnt += 1class Plane(Process):    def __init__(self,name):        Process.__init__(self,name)    def execute(self):        # landing        start = now()        yield request, self, runways        finish = now()        yield hold, self, T_RUNWAY        yield release, self, runways        mrwl.observe(finish - start)        am.add()        # gate        start = now()        yield request, self, gates        finish = now()        am.sub()        yield hold, self, T_GATES        yield release, self, gates        mg.observe(finish - start)        am.add()        # take off        start = now()        yield request, self, runways        finish = now()        am.sub()        yield hold, self, T_RUNWAY        yield release, self, runways        mrwt.observe(finish - start)class AirportMonitor:    def __init__(self):        self.cnt = 0        self.max = 0    def add(self):        self.cnt += 1        if (self.cnt > self.max):            self.max = self.cnt    def sub(self):        self.cnt -= 1initialize()r = random.Random()am = AirportMonitor()mrwl = Monitor("runway landing")mrwt = Monitor("runway takeoff")mg = Monitor("gate")runways = Resource(NUM_RUNWAY)gates = Resource(NUM_GATE)pg = PlaneGenerator('Source')activate(pg, pg.generate(), 0.0)simulate(until=24*60)print "total planes generated: " + str(pg.cnt)print "average waiting time for runway (landing): " + str(mrwl.mean())print "average waiting time for runway (takeoff): " + str(mrwt.mean())print "average waiting time for gate: " + str(mg.mean())print "max planes waiting for gate/runway: " + str(am.max)