Yield hold until other line > 0

Question

I'm using SimPy 2.3 and have a process which generates customers at an ATM at a random rate and another process which serves customers at a random rate. When the line is empty I want the ATM to wait for the next customer before doing anything else.

Here's some code

class ATM(Process):
    def Run(self):
        while 1:
            if self.atm_line.customers is 0:
                yield hold, wait for self.atm_line.customers != 0 # this is the line I'm stuck on
            yield hold, self, random.expovariate(.1)
            self.atm_line.customers -= 1

class ATM_Line(Process):
    def __init__(self):
        self.customers = 0
        Process.__init__(self)

    def Run(self):
        while 1:
            yield hold, self, random.expovariate(.1)
            self.customers += 1

initialize()
a = ATM()
a.atm_line = ATM_Line()
activate(a, a.Run())
activate(a.atm_line, a.atm_line.Run())
simulate(until=10000)

What is a good way to do this?

Answer 1

I was able to solve this problem using yield waitevent and signals. The working code is below.

from SimPy.Simulation import *
from random import Random, expovariate, uniform

class ATM(Process):
    def Run(self):
        while 1:
            if self.atm_line.customers is 0:
                yield waitevent, self, self.atm_line.new_customer
            yield hold, self, random.expovariate(.05)
            self.atm_line.customers -= 1

class ATM_Line(Process):
    def __init__(self):
        self.customers = 0
        self.new_customer = SimEvent()
        Process.__init__(self)

    def Run(self):
        while 1:
            yield hold, self, random.expovariate(.07)
            self.new_customer.signal()
            self.customers += 1

initialize()
a = ATM()
a.atm_line = ATM_Line()
activate(a, a.Run())
activate(a.atm_line, a.atm_line.Run())
simulate(until=50)