Simpy: increase store count only if sub stores have items

I'm trying to model a manufacturing line where some machines depends on specific items produced by other machines and can't start its job until previous ones put the items into their stores.

For example:

We have 4 Machines (A, B, C, D), Machine A has a Store A* (B,C,D Stores correspondingly).
Machine A can start work only if there's an item in Store A*. Item in A* consists of three items from B*, C*, D* Stores.

Is there any way to increase count in Store A* only if Stores B*, C*, D* have at least one item?

Solution

You need to use the env.all_of event to wait for a set of events to finish. In this case, 3 get() events from the three ouput queues from 3 machines. see the example

"""
Simple model how one machine does not start until it has multiple inputs

Programmer: Michael R. Gibbs
"""

import simpy
import random

class Entity(object):
    pass

def imput_process(env, id_pre, time_func, out_q):
    """
    simple loop to generate output entities
    to be consumed by the out process
    """
    
    cnt = 0

    while True:
        yield env.timeout(time_func())

        cnt += 1
        ent = Entity()
        ent.id = id_pre + ' ' + str(cnt)

        out_q.put(ent)
        print(f'{env.now} made entity {ent.id}')

def out_process(env, a_q, b_q, c_q, time_func):
    """
    waits until has a entity from each of the queues
    then do some processing
    """

    while True:

        # make the requests in parrelle
        # note no yields here
        req_a = a_q.get()
        req_b = b_q.get()
        req_c = c_q.get()

        # now us a all_of to wait for all 
        # the requests to finish
        response = yield env.all_of([req_a, req_b, req_c])

        # response is a dict where key is the reqest and value is 
        # what is what would be returned by a yield to the request

        ent_a = response[req_a]
        ent_b = response[req_b]
        ent_c = response[req_c]

        print(f'{env.now} out has entites {ent_a.id}, {ent_b.id}, {ent_c.id}')

        yield env.timeout(time_func())

        print(f'{env.now} out process completed')

# bootup

env = simpy.Environment()

store_a = simpy.Store(env)
store_b = simpy.Store(env)
store_c = simpy.Store(env)

env.process(imput_process(env, 'A', lambda : random.triangular(1,1,5), store_a))
env.process(imput_process(env, 'B', lambda : random.triangular(1,3,5), store_b))
env.process(imput_process(env, 'C', lambda : random.triangular(1,5,5), store_c))

env.process(out_process(env, store_a, store_b, store_c, lambda : random.triangular(1,2,4)))

env.run(100)