假设我有一个可以使用async for传递的异步迭代，那么我怎样才能将它映射并过滤到新的异步迭代器？以下代码是对我如何使用同步迭代执行相同操作的调整不起作用，因为yield内不允许async def。
async def mapfilter(aiterable, p, func):
async for payload in aiterable:
if p(payload):
# This part isn't allowed, but hopefully it should be clear
# what I'm trying to accomplish.
yield func(payload)
答案 0 :(得分：5)
支持https://msdn.microsoft.com/en-us/library/4k7zyeba.aspx的recently published PEP draft (PEP 525)建议允许异步生成器使用与您相同的语法。
同时，如果您不想处理异步迭代器样板，也可以在注释中使用scheduled for Python 3.6中提到的asyncio_extras库。
@async_generator
async def mygenerator(websites):
for website in websites:
page = await http_fetch(website)
await yield_async(page)
async def fetch_pages():
websites = ('http://foo.bar', 'http://example.org')
async for sanitized_page in mygenerator(websites):
print(sanitized_page)
还有the docs支持yield from构造。
答案 1 :(得分：3)
你can't在协同程序中使用yield。要实现您的想法，我看到的只是实现Asynchronous Iterator。如果我是对的，那就是：
class MapFilter:
def __init__(self, aiterable, p, func):
self.aiterable = aiterable
self.p = p
self.func = func
async def __aiter__(self):
return self
async def __anext__(self):
while True:
payload = await self.aiterable.__anext__() # StopAsyncIteration would be raise here on no new values
if self.p(payload):
return self.func(payload)
让我们测试一下。这是帮助arange类的完整示例（我从here获取）：
import asyncio
class arange:
def __init__(self, n):
self.n = n
self.i = 0
async def __aiter__(self):
return self
async def __anext__(self):
i = self.i
self.i += 1
if self.i <= self.n:
await asyncio.sleep(0) # insert yield point
return i
else:
raise StopAsyncIteration
class MapFilter:
def __init__(self, aiterable, p, func):
self.aiterable = aiterable
self.p = p
self.func = func
async def __aiter__(self):
return self
async def __anext__(self):
while True:
payload = await self.aiterable.__anext__()
if self.p(payload):
return self.func(payload)
async def main():
aiterable = arange(5)
p = lambda x: bool(x>2)
func = lambda x: x*2
async for i in MapFilter(aiterable, p, func):
print(i)
if __name__ == "__main__":
loop = asyncio.get_event_loop()
loop.run_until_complete(main())
输出：
6
8
答案 2 :(得分：0)
https://gist.github.com/dvdotsenko/d8e0068775ac04b58993f604f122284f
适用于Python 3.6+的异步map和filter实现，专门设计为无序返回子任务，以先完成者为准。
from collections import deque
from typing import Any, Callable, Collection, AsyncIterator, Iterator, Union
async def _next(gg):
# repackaging non-asyncio next() as async-like anext()
try:
return next(gg)
except StopIteration:
raise StopAsyncIteration
async def _aionext(gg):
# there is no anext() :(
return await gg.__anext__()
async def map_unordered(fn:Callable, args:Union[Iterator,Collection,AsyncIterator], maxsize=None):
"""
Async generator yielding return values of resolved invocations
of `fn` against arg in args list
Arguments are consumed and fed to callable in the order they are presented in args.
Results are yielded NOT in order of args. Earliest done is yielded.
If `size` is specified, worker tasks pool is constrained to that size.
This is asyncio equivalent of Gevent's `imap_unordered(fn, args_iterable, pool_size)`
http://www.gevent.org/api/gevent.pool.html#gevent.pool.Group.imap_unordered
`args` may be Async Iterator or regular Iterator.
Thus, you can chain `map_unordered` as `args` for another `map_unordered`
Because this is an async generator, cannot consume it as regular iterable.
Must use `async for`.
Usage example:
# note NO await in this assignment
gen = map_unordered(fn, arguments_iter, maxsize=3)
async for returned_value in gen:
yield returned_value
"""
if maxsize == 0:
raise ValueError(
'Argument `maxsize` cannot be set to zero. '
'Use `None` to indicate no limit.'
)
# Make args list consumable like a generator
# so repeated islice(args, size) calls against `args` move slice down the list.
if hasattr(args, '__anext__'):
n = _aionext
elif hasattr(args, '__next__'):
n = _next
else:
args = iter(args)
n = _next
have_args = True # assumed. Don't len(args).
pending_tasks = deque()
while have_args or len(pending_tasks):
try:
while len(pending_tasks) != maxsize:
arg = await n(args)
pending_tasks.append(
asyncio.Task(fn(arg))
)
except StopAsyncIteration:
have_args = False
if not len(pending_tasks):
return
done, pending_tasks = await asyncio.wait(pending_tasks, return_when=asyncio.FIRST_COMPLETED)
pending_tasks = deque(pending_tasks)
for task in done:
yield await task # await converts task object into its return value
async def _filter_wrapper(fn, arg):
return (await fn(arg)), arg
async def _filter_none(arg):
return not (arg is None)
async def filter_unordered(fn:Union[Callable,None], args:Union[Iterator,Collection,AsyncIterator], maxsize=None):
"""
Async filter generator yielding values of `args` collection that match filter condition.
Like python's native `filter([Callable|None], iterable)` but:
- allows iterable to be async iterator
- allows callable to be async callable
- returns results OUT OF ORDER - whichever passes filter test first.
Arguments are consumed and fed to callable in the order they are presented in args.
Results are yielded NOT in order of args. Earliest done and passing the filter condition is yielded.
If `maxsize` is specified, worker tasks pool is constrained to that size.
This is inspired by Gevent's `imap_unordered(fn, args_iterable, pool_size)`
http://www.gevent.org/api/gevent.pool.html#gevent.pool.Group.imap_unordered
Because this is an async generator, cannot consume it as regular iterable.
Must use `async for`.
Usage example:
# note NO await in this assignment
gen = filter_unordered(fn, arguments_iter, maxsize=3)
async for returned_value in gen:
yield returned_value
"""
if maxsize == 0:
raise ValueError(
'Argument `maxsize` cannot be set to zero. '
'Use `None` to indicate no limit.'
)
if hasattr(args, '__anext__'):
n = _aionext
elif hasattr(args, '__next__'):
n = _next
else:
args = iter(args)
n = _next
if fn is None:
fn = _filter_none
have_args = True # assumed. Don't len(args).
pending_tasks = deque()
while have_args or len(pending_tasks):
try:
while len(pending_tasks) != maxsize:
arg = await n(args)
pending_tasks.append(
asyncio.Task(_filter_wrapper(fn,arg))
)
except StopAsyncIteration:
have_args = False
if not len(pending_tasks):
return
done, pending_tasks = await asyncio.wait(pending_tasks, return_when=asyncio.FIRST_COMPLETED)
pending_tasks = deque(pending_tasks)
for task in done:
filter_match, arg = await task
if filter_match:
yield arg
与Gevent的imap_unordered类似，但与Gevent的版本不同，它还允许可迭代的args成为异步值生成器。意味着您可以将它们链接起来。
给出：
async def worker(seconds):
print('> Start wait', seconds)
await asyncio.sleep(seconds)
print('< End wait', seconds)
return seconds
async def to_aio_gen(ll):
for e in ll:
yield e
async def test_map(ll, size=None):
t = time.time()
async for v in map_unordered(worker, ll, maxsize=size):
print('-- elapsed second', round(time.time() - t, 1), ' received value', v)
ll = [
0.2,
0.4,
0.8,
1.2,
1.1,
0.3,
0.6,
0.9,
]
测试输出：
非异步可迭代，池大小= 3
>>> asyncio.run(test_map(ll, 3))
> Start wait 0.2
> Start wait 0.4
> Start wait 0.8
< End wait 0.2
-- elapsed second 0.2 received value 0.2
> Start wait 1.2
< End wait 0.4
-- elapsed second 0.4 received value 0.4
> Start wait 1.1
< End wait 0.8
-- elapsed second 0.8 received value 0.8
> Start wait 0.3
< End wait 0.3
-- elapsed second 1.1 received value 0.3
> Start wait 0.6
< End wait 1.2
-- elapsed second 1.4 received value 1.2
> Start wait 0.9
< End wait 1.1
-- elapsed second 1.5 received value 1.1
< End wait 0.6
-- elapsed second 1.7 received value 0.6
< End wait 0.9
-- elapsed second 2.3 received value 0.9
异步迭代器作为arg列表，池大小= 3，过滤器
async def more_than_half(v):
await asyncio.sleep(v)
return v > 0.5
>>> asyncio.run(filter_unordered(more_than_half, aio_gen(ll), 3))
-- elapsed second 0.8 received value 0.8
-- elapsed second 1.4 received value 1.2
-- elapsed second 1.5 received value 1.1
-- elapsed second 1.7 received value 0.6
-- elapsed second 2.3 received value 0.9