Example of unfair scheduling in CPython's condition variable implementation

unfiar_scheduling.py

from threading import Condition, Thread
import time

# This is a sample implementation of something like an object pool. It uses
# a condition variable to ensure that at most `counter` threads are allowed
# to execute at a time. 
THREAD_COUNT = 10
counter = 1
 
cnd = Condition()

 
def work(tid):
    global counter
    for i in range(1000):
        # Wait for counter to be positive so we are allowed to run 
        with cnd:
            cnd.wait_for(lambda: counter > 0)
            counter -= 1

        print("thread", tid, "running")
        # Sample logic to run in the critical section.
        time.sleep(0.05)

        # Give up counter. Ideally this would allow someone else to run. In practice
        # however we will immediately loop and run again before anyone else gets
        # the chance.
        with cnd:
            counter += 1
            cnd.notify()


threads = [Thread(target=work, args=(tid,)) for tid in range(THREAD_COUNT)]
for thread in threads:
    thread.start()
for thread in threads:
    thread.join()