python async with和async for的使用

class AsyncContextManager:
  async def __aenter__(self):
    await log('entering context')

  async def __aexit__(self, exc_type, exc, tb):
    await log('exiting context')

async with EXPR as VAR:
  BLOCK

mgr = (EXPR)
aexit = type(mgr).__aexit__
aenter = type(mgr).__aenter__(mgr)
exc = True

VAR = await aenter
try:
  BLOCK
except:
  if not await aexit(mgr, *sys.exc_info()):
    raise
else:
  await aexit(mgr, None, None, None)

async def commit(session, data):
  ...

  async with session.transaction():
    ...
    await session.update(data)
    ...

async with lock:
  ...

with (yield from lock):
  ...

class AsyncIterable:
  def __aiter__(self):
    return self

  async def __anext__(self):
    data = await self.fetch_data()
    if data:
      return data
    else:
      raise StopAsyncIteration

  async def fetch_data(self):
    ...

async for TARGET in ITER:
  BLOCK
else:
  BLOCK2

iter = (ITER)
iter = type(iter).__aiter__(iter)
running = True
while running:
  try:
    TARGET = await type(iter).__anext__(iter)
  except StopAsyncIteration:
    running = False
  else:
    BLOCK
else:
  BLOCK2

async for data in cursor:
  ...

class Cursor:
  def __init__(self):
    self.buffer = collections.deque()

  async def _prefetch(self):
    ...

  def __aiter__(self):
    return self

  async def __anext__(self):
    if not self.buffer:
      self.buffer = await self._prefetch()
      if not self.buffer:
        raise StopAsyncIteration
    return self.buffer.popleft()

async for row in Cursor():
  print(row)
which would be equivalent to the following code:

i = Cursor().__aiter__()
while True:
  try:
    row = await i.__anext__()
  except StopAsyncIteration:
    break
  else:
    print(row)

class AsyncIteratorWrapper:
  def __init__(self, obj):
    self._it = iter(obj)

  def __aiter__(self):
    return self

  async def __anext__(self):
    try:
      value = next(self._it)
    except StopIteration:
      raise StopAsyncIteration
    return value

async for letter in AsyncIteratorWrapper("abc"):
  print(letter)

def g1():
  yield from fut
  return 'spam'

def g2():
  yield from fut
  raise StopIteration('spam')

async def a1():
  await fut
  raise StopIteration('spam')