当在greenlets（除了主greenlet）中调用queue.get时，gevent队列不会引发Empty？

Question

I am very new to gevent and my question might even be very naive, but the when I run following code I get queue empty exception.

case 1 makes sense

import gevent
from gevent import monkey
monkey.patch_all()

q = gevent.queue.Queue(maxsize=10)

try:
    q.get(timeout=1)
except gevent.queue.Empty:
    print("main greenlet queue empty")

but if I run following code I do not get any timeout and code just never invokes any exceptions and I never enter the exception handeling block, in fact code doesn't even wait for 1 sec to fetch data from queue. I just executes immediately with the shown output.

case 2

import gevent
from gevent import monkey
monkey.patch_all()

q = gevent.queue.Queue(maxsize=10)

def worker(idx):
    print("worker", idx)
    try:
        q.get(timeout=1)
    except gevent.queue.Empty:
        print("worker", idx, "queue empty")

workers = [gevent.spawn(worker, i) for i in range(1)]

this outputs:

worker 0

However, if I call get in the main greenlet with a call to queue.get in both child and main greenlet like following, the behaviour now changes back to normal.

case 3

import gevent
from gevent import monkey
monkey.patch_all()

q = gevent.queue.Queue(maxsize=10)

def worker(idx):
    print("worker", idx)
    try:
        q.get(timeout=1)
    except gevent.queue.Empty:
        print("worker", idx, "queue empty")

workers = [gevent.spawn(worker, i) for i in range(1)]

try:
    q.get(timeout=1)
except gevent.queue.Empty:
    print("main greenlet queue empty")

I get following output.

worker 0
main greenlet queue empty
worker 0 queue empty

I fail to understand that why does case 2 not raise an exception, can some one please explain to me what am I doing wrong?

I am using python 3.7 and gevent '21.1.2'

Answer 1

In your second case, you are starting greenlets (the interpreter exits after). Adding joinall() makes your code wait for the spawned greenlets to return:

q = gevent.queue.Queue(maxsize=10)

def worker(idx):
    print("worker", idx)
    try:
        q.get(timeout=1)
    except gevent.queue.Empty:
        print("worker", idx, "queue empty")


print(q.empty())
workers = [gevent.spawn(worker, i) for i in range(1)]
# waiting for them to finish, works as expected!
gevent.joinall(workers)

Out:

True
worker 0
worker 0 queue empty