Global array returns empty despite having an element

Question

I have what might be a quite simple problem, but I still cannot solve it.

I have the following code:

list_of_conditions = []  
def function_1():  
    time.sleep(60)  
    global list_of_conditions
    condition=list_of_conditions.pop()  
    condition.notify()  
    print("F1: Done")

def function_2():
    condition = Condition()  
    global list_of_conditions
    list_of_conditions.append(condition)
    with condition:  
       p1=Process(target=function_1, daemon=True)  
       p1.start()  
       condition.wait()
       print("Finished")

I think it is quite clear what I want to accomplish with the condition, but I have the problem that the global array is not working. I keep getting the error message:

IndexError: pop from empty list

I dont get it why the list is empty even though I have declare it as global.

Because you're using Multiprocessing. Processes do *not* share state. You have to share it yourself. In this instance you can just pass it in when you start function_1: there's no reason for global state. If the logic is more complicated, do proper producer/consumer with a queue. — 2e0byo, Oct 26 '22 at 15:11
Quick aside: the `global` statement is entirely unnecessary here. You don't need `global` to refer to a global variable. You only need it if you are going to assign to it. Since you only call functions on `list_of_conditions` you're only referring to it. — Steven Rumbalski, Oct 26 '22 at 15:15
To underline @StevenRumbalski's point: appending to (and popping from) a list is *not* the same as *binding a name to a new object*, which is what happens when you do something like `global x; x += 1`: in this case the global name 'x' is set up to point to the number 1 greater than x. Had you used `+=[condition]` you *would* have needed global (in `function_2`). Modifying the *contents* of a mutable is not the same as re-assigning the *name* to point to a *new* object, even if the end result is the same. — 2e0byo, Oct 26 '22 at 15:22

score 1 · Accepted Answer · answered Oct 26 '22 at 15:17

I dont get it why the list is empty

Because you're using Multiprocessing. Processes do not share mutable state: each Process starts with a blank slate, i.e. it sees the module as it was at intial evaluation. Sharing mutable state in multithreading/multiprocessing paradigms is problematic: what if two threads try to modify the same state?

Thus if you want to share state, you have to do it explicitly. In this instance you could use a queue, and set up a pool of consumers waiting for Condition()s to be sent them down the queue.

However if the logic as presented here is all that goes on, just modify function_1 to take the Condition() as an argument, and pass it through when you start the process:

def handle_condition(condition: Condition):
    ...

def call_handler():
    condition = Condition()
    # note that args is a tuple
    Process(target=handle_condition, args=(condition,)).start()

With that said, your current logic doesn't make much sense---if you're going to stop and wait for every process to run every time, why not just process in the same thread?

Thank you for your explanation. That solved one problem, but another one appeared :D If you have time, please take a look: https://stackoverflow.com/questions/74222512/condition-objects-should-only-be-shared-between-processes-through-inheritance — Tricker Macedonia, Oct 27 '22 at 13:54

Global array returns empty despite having an element

1 Answers1