Typeerror: 'async for' requires an object with __aiter__ method, got list

A Python generator is any function containing one or more yield expressions:

def func(): # a function return def genfunc(): # a generator function yield

We propose to use the same approach to define asynchronous generators:

async def coro(): # a coroutine function await smth() async def asyncgen(): # an asynchronous generator function await smth() yield 42

The result of calling an asynchronous generator function is an asynchronous generator object, which implements the asynchronous iteration protocol defined in PEP 492.

It is a SyntaxError to have a non-empty return statement in an asynchronous generator.

PEP 492 requires an event loop or a scheduler to run coroutines. Because asynchronous generators are meant to be used from coroutines, they also require an event loop to run and finalize them.

Asynchronous generators can have try..finally blocks, as well as async with. It is important to provide a guarantee that, even when partially iterated, and then garbage collected, generators can be safely finalized. For example:

async def square_series(con, to): async with con.transaction(): cursor = con.cursor( 'SELECT generate_series(0, $1) AS i', to) async for row in cursor: yield row['i'] ** 2 async for i in square_series(con, 1000): if i == 100: break

The above code defines an asynchronous generator that uses async with to iterate over a database cursor in a transaction. The generator is then iterated over with async for, which interrupts the iteration at some point.

The square_series() generator will then be garbage collected, and without a mechanism to asynchronously close the generator, Python interpreter would not be able to do anything.

To solve this problem we propose to do the following:

1. Implement an

   aclose

   method on asynchronous generators returning a special awaitable. When awaited it throws a

   GeneratorExit

   into the suspended generator and iterates over it until either a

   GeneratorExit

   or a

   StopAsyncIteration

   occur.

   This is very similar to what the close() method does to regular Python generators, except that an event loop is required to execute aclose().

2. Raise a

   RuntimeError

   , when an asynchronous generator executes a

   yield

   expression in its

   finally

   block (using

   await

   is fine, though):

   async def gen(): try: yield finally: await asyncio.sleep(1) # Can use 'await'. yield # Cannot use 'yield', # this line will trigger a # RuntimeError.

3. Add two new methods to the

   sys

   module:

   set_asyncgen_hooks()

   and

   get_asyncgen_hooks()

   .

The idea behind sys.set_asyncgen_hooks() is to allow event loops to intercept asynchronous generators iteration and finalization, so that the end user does not need to care about the finalization problem, and everything just works.

sys.set_asyncgen_hooks() accepts two arguments:

• firstiter

  : a callable which will be called when an asynchronous generator is iterated for the first time.

• finalizer

  : a callable which will be called when an asynchronous generator is about to be GCed.

When an asynchronous generator is iterated for the first time, it stores a reference to the current finalizer.

When an asynchronous generator is about to be garbage collected, it calls its cached finalizer. The assumption is that the finalizer will schedule an aclose() call with the loop that was active when the iteration started.

For instance, here is how asyncio is modified to allow safe finalization of asynchronous generators:

# asyncio/base_events.py class BaseEventLoop: def run_forever(self): ... old_hooks = sys.get_asyncgen_hooks() sys.set_asyncgen_hooks(finalizer=self._finalize_asyncgen) try: ... finally: sys.set_asyncgen_hooks(*old_hooks) ... def _finalize_asyncgen(self, gen): self.create_task(gen.aclose())

The second argument, firstiter, allows event loops to maintain a weak set of asynchronous generators instantiated under their control. This makes it possible to implement “shutdown” mechanisms to safely finalize all open generators and close the event loop.

sys.set_asyncgen_hooks() is thread-specific, so several event loops running in parallel threads can use it safely.

sys.get_asyncgen_hooks() returns a namedtuple-like structure with firstiter and finalizer fields.

The asyncio event loop will use sys.set_asyncgen_hooks() API to maintain a weak set of all scheduled asynchronous generators, and to schedule their aclose() coroutine methods when it is time for generators to be GCed.

To make sure that asyncio programs can finalize all scheduled asynchronous generators reliably, we propose to add a new event loop coroutine method loop.shutdown_asyncgens(). The method will schedule all currently open asynchronous generators to close with an aclose() call.

After calling the loop.shutdown_asyncgens() method, the event loop will issue a warning whenever a new asynchronous generator is iterated for the first time. The idea is that after requesting all asynchronous generators to be shutdown, the program should not execute code that iterates over new asynchronous generators.

An example of how shutdown_asyncgens coroutine should be used:

try: loop.run_forever() finally: loop.run_until_complete(loop.shutdown_asyncgens()) loop.close()

The object is modeled after the standard Python generator object. Essentially, the behaviour of asynchronous generators is designed to replicate the behaviour of synchronous generators, with the only difference in that the API is asynchronous.

The following methods and properties are defined:

1. agen.__aiter__()

   : Returns

   agen

   .

2. agen.__anext__()

   : Returns an awaitable, that performs one asynchronous generator iteration when awaited.

3. agen.asend(val)

   : Returns an awaitable, that pushes the

   val

   object in the

   agen

   generator. When the

   agen

   has not yet been iterated,

   val

   must be

   None

   .

   Example:

   async def gen(): await asyncio.sleep(0.1) v = yield 42 print(v) await asyncio.sleep(0.2) g = gen() await g.asend(None) # Will return 42 after sleeping # for 0.1 seconds. await g.asend('hello') # Will print 'hello' and # raise StopAsyncIteration # (after sleeping for 0.2 seconds.)

4. agen.athrow(typ,

   [val,

   [tb]])

   : Returns an awaitable, that throws an exception into the

   agen

   generator.

   Example:

   async def gen(): try: await asyncio.sleep(0.1) yield 'hello' except ZeroDivisionError: await asyncio.sleep(0.2) yield 'world' g = gen() v = await g.asend(None) print(v) # Will print 'hello' after # sleeping for 0.1 seconds. v = await g.athrow(ZeroDivisionError) print(v) # Will print 'world' after $ sleeping 0.2 seconds.

5. agen.aclose()

   : Returns an awaitable, that throws a

   GeneratorExit

   exception into the generator. The awaitable can either return a yielded value, if

   agen

   handled the exception, or

   agen

   will be closed and the exception will propagate back to the caller.

6. agen.__name__

   and

   agen.__qualname__

   : readable and writable name and qualified name attributes.

7. agen.ag_await

   : The object that

   agen

   is currently awaiting on, or

   None

   . This is similar to the currently available

   gi_yieldfrom

   for generators and

   cr_await

   for coroutines.

8. agen.ag_frame

   ,

   agen.ag_running

   , and

   agen.ag_code

   : defined in the same way as similar attributes of standard generators.

StopIteration and StopAsyncIteration are not propagated out of asynchronous generators, and are replaced with a RuntimeError.

Asynchronous generator object (PyAsyncGenObject) shares the struct layout with PyGenObject. In addition to that, the reference implementation introduces three new objects:

1. PyAsyncGenASend

   : the awaitable object that implements

   __anext__

   and

   asend()

   methods.

2. PyAsyncGenAThrow

   : the awaitable object that implements

   athrow()

   and

   aclose()

   methods.

3. _PyAsyncGenWrappedValue

   : every directly yielded object from an asynchronous generator is implicitly boxed into this structure. This is how the generator implementation can separate objects that are yielded using regular iteration protocol from objects that are yielded using asynchronous iteration protocol.

PyAsyncGenASend and PyAsyncGenAThrow are awaitables (they have __await__ methods returning self) and are coroutine-like objects (implementing __iter__, __next__, send() and throw() methods). Essentially, they control how asynchronous generators are iterated:

The proposal is fully backwards compatible.

In Python 3.5 it is a SyntaxError to define an async def function with a yield expression inside, therefore it’s safe to introduce asynchronous generators in 3.6.