Trying to understand event schedulingenv._queue with regards to SimPy's FilterStore put and get events - Stack Overflow

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Some context about the original simulation. I have 2 processes P1 and P2 which are based off the same function. This function looks through FilterStore.items first to "screen" for an item with the lowest value of a certain attribute, and then performs a get with the exact value of that attribute which will get a desired item. If the store was empty, then it will simply have a parameterless get which yields the first item put into the store. If you need an analogy, one could be consumers wanting to buy a GPU with the lowest cost, but if there was no GPU in stock, they would just get their hands on whatever GPU becomes available first.

Now, suppose the store was empty at first, so when P1 executes, it yields for a parameterless get. Subsequently, store.put(item) and then P2 is in the event queue at the same time in that order. At the end, P1 gets the item ... but somehow P2 is yielding for a filtered get instead of a parameterless one!

Here is a sample code, and I have tagged the env._queue print statements with a number before it so that I can reference to the output and the order as well. Note that foo() and bar() are actually the same functions, but represent P1 and P2. baz() is a method to perform put.

import simpy

class something:
    def __init__(self, dist):
        self.dist = dist

env = simpy.Environment()
store = simpy.FilterStore(env)

def foo():
    min_dist = float('inf')
    for item in store.items:
        if item.dist < min_dist:
            min_dist = item.dist

    if min_dist != float('inf'):
        # Will never execute for this example
        item = yield store.get(lambda item: item.dist == min_dist)
    else:
        item = store.get()
        item.callbacks.append(lambda _: print("7) foo get has completed " + str(env._queue)))
        print(f"foo is now in the else-block, foo's get process is {item}")
        print(f'1) {env._queue}')
        item = yield item
        print(f"8) {env._queue}") # Foo got the item!


def bar():
    min_dist = float('inf')
    for item in store.items:
        if item.dist < min_dist:
            min_dist = item.dist

    print(f'4) {env._queue}')

    if min_dist != float('inf'):
        item = store.get(lambda item: item.dist == min_dist)
        print(f"bar is now in the if-block, bar's get process is {item}")
        print(f'5) {env._queue}')
        item.callbacks.append(lambda _: print("bar get complete " + str(env._queue)))
        item = yield item
        print(f"bar gets the item, queue = {env._queue}")

    else:
        # This was the branch I expected to execute, but it did not
        item = yield store.get()

def baz():
    print(f'2) {env._queue}')
    print(f"baz has not put in the object, store items = {store.items}")
    put = store.put(something(2))
    put.callbacks.append(lambda _: print(f"6) baz first put has completed" + str(env._queue)))
    print(f"baz has put in the object, store items = {store.items}")
    print(f'3) {env._queue}')
    yield env.timeout(50)
    print(f'9) {env._queue}, this is after the env.timeout. At this point, bar is still waiting')
    put = store.put(something(2))
    print(f"10) {env._queue}, final put is {put}")
    put.callbacks.append(lambda _: print(f"11) final put {put} has complete " + str(env._queue)))

env.process(foo())
env.process(baz())
env.process(bar())
print(f'0) {env._queue}')

env.run()

Some questions I have regarding the output, which can be gotten by running the sample code.

1. In statement 3 and the print statement above it, a StorePut() was triggered and placed after the Initialize() in the event queue. However, the item is already in the store before the actual StorePut() has been processed. What purpose does StorePut() then do? Does it notify the store to trigger any processes waiting for an item?

2. In statement 5 after bar has entered the wrong branch and performed a filtered get. At this point, a FilterStoreGet() object belonging to foo is triggered and placed in the queue. Can I think of this get by bar as additionally notifying the store to perform a check and triggers the original request by foo as an item is now in the store?

3. In statement 11 (within the callback), baz should have terminated, but how come there is a Process(baz) object in the queue, especially since the put being processed should imply that baz has been processed? Notice that all other callback statements do not have it.

4. Finally, with regards to the first question, is there a way for me to modify put behaviour, so that the item is NOT in the stall until the event is processed or if that is not possible, is there a way to "delay" this so that bar executes before the first put event? With this, P2 or bar can go down the intended branch.

Thank you all in advance for any insight that is provided!

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