async-generator-by-generator-only.py

from datetime import datetime
import asyncio

# The code is based on this link:
# https://www.facebook.com/photo.php?fbid=1723296854349009&set=p.1723296854349009&type=3&theater
# And of course, the output is exactly the same.
# For conveniency, I attached the output at the end of this file.

# ----- begin of polyfill -----
Await = 'await'
Yield = 'yield'
YieldFrom = 'yield_from'
AsyncFor = 'async_for'

def await_(v): return (Await, v)
def emit_(v): return (Yield, v)
def emit_from_(v): return (YieldFrom, v)
def async_for_(g, f): return (AsyncFor, (g, f))

def noop(x): pass

# the generator resolve function
def resolveAG(g, emit, done):
  def step(v):
    try:
      if type(v) == asyncio.Task:
        (t, v) = g.send(v.result())
      else:
        (t, v) = g.send(None)

      if t == Await:
        future = asyncio.ensure_future(v)
        future.add_done_callback(step)
      elif t == Yield:
        emit(v)
        step(None)

      elif t == YieldFrom: # like flatten
        resolveAG(v, emit, step)
      elif t == AsyncFor: # like flatMap
        (g2, f2) = v

        def pipe(v):
          g3 = f2(v)
          resolveAG(g3, emit, noop)

        resolveAG(g2, pipe, step)
    except StopIteration:
      done(None)
      return

  step(None)

# the runner
def runAG(g, cb):
  future = asyncio.Future()
  resolveAG(g, cb, future.set_result)
  return future

# ----- end of polyfill -----

# ----- begin of test code -----
def counter(n):
  for i in range(n):
    a = yield await_(asyncio.sleep(1, "log from {}th async-gen".format(i)))
    yield emit_(a)

    def nested(j):
      yield emit_("nested number %s" % j)

    yield async_for_(counter(i - 1), nested)

def printer(n):
  return runAG(counter(n), lambda i:
      print('{} {}'.format(i, datetime.now())))

loop = asyncio.get_event_loop()
loop.run_until_complete(printer(5))

# Output

# log from 0th async-gen 2017-05-21 02:12:01.756030
# log from 1th async-gen 2017-05-21 02:12:02.757923
# log from 2th async-gen 2017-05-21 02:12:03.761649
# nested number log from 0th async-gen 2017-05-21 02:12:04.766404
# log from 3th async-gen 2017-05-21 02:12:05.771796
# nested number log from 0th async-gen 2017-05-21 02:12:06.774977
# nested number log from 1th async-gen 2017-05-21 02:12:07.779314
# log from 4th async-gen 2017-05-21 02:12:08.783343
# nested number log from 0th async-gen 2017-05-21 02:12:09.788776
# nested number log from 1th async-gen 2017-05-21 02:12:10.791849
# nested number log from 2th async-gen 2017-05-21 02:12:11.797282
# nested number nested number log from 0th async-gen 2017-05-21 02:12:12.800406