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From: Marko Rauhamaa <marko@pacujo.net>
Newsgroups: comp.lang.python
Subject: Emperor's New Coroutines?
Date: Mon, 07 Jul 2014 12:42:28 +0300
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The asyncio module comes with coroutine support. Investigating the topic
on the net reveals that FSM's are for old people and the brave new world
uses coroutines. Unfortunately, all examples I could find seem to be
overly simplistic, and I'm left thinking coroutines have few practical
uses in network programming.

PEP-380 refers to the classic Dining Philosophers: <URL: http://www.cos
c.canterbury.ac.nz/greg.ewing/python/yield-from/yf_current/Examples/Sch
eduler/philosophers.py> (reproduced below).

Suppose I want to fix the protocol by supplying an "attendant" object
that periodically tells the philosophers to drop whatever they are doing
and start thinking. So I would add:

   schedule(attendant(..., [ plato, socrates, euclid ]))

but how would I modify the philosopher code to support such master
resets?


Marko

========================================================================
from scheduler import *

class Utensil:

  def __init__(self, id):
    self.id = id
    self.available = True
    self.queue = []

  def acquire(self):
    #print "--- acquire", self.id, "avail", self.available
    if not self.available:
      block(self.queue)
      yield
    #print "--- acquired", self.id
    self.available = False

  def release(self):
    #print "--- release", self.id
    self.available = True
    unblock(self.queue)

def philosopher(name, lifetime, think_time, eat_time, left_fork,
right_fork):
  for i in range(lifetime):
    for j in range(think_time):
      print(name, "thinking")
      yield
    print(name, "waiting for fork", left_fork.id)
    yield from left_fork.acquire()
    print(name, "acquired fork", left_fork.id)
    print(name, "waiting for fork", right_fork.id)
    yield from right_fork.acquire()
    print(name, "acquired fork", right_fork.id)
    for j in range(eat_time):
      # They're Python philosophers, so they eat spam rather than
      spaghetti
      print(name, "eating spam")
      yield
    print(name, "releasing forks", left_fork.id, "and", right_fork.id)
    left_fork.release()
    right_fork.release()
  print(name, "leaving the table")

forks = [Utensil(i) for i in range(3)]
schedule(philosopher("Plato", 7, 2, 3, forks[0], forks[1]), "Plato")
schedule(philosopher("Socrates", 8, 3, 1, forks[1], forks[2]),
"Socrates")
schedule(philosopher("Euclid", 5, 1, 4, forks[2], forks[0]), "Euclid")
run()