API documentation

Async generators

In Python 3.6+, you can write a native async generator like this:

async def load_json_lines(stream_reader):
    async for line in stream_reader:
        yield json.loads(line)

Here’s the same thing written with this library, which works on Python 3.5+:

from async_generator import async_generator, yield

async def load_json_lines(stream_reader):
    async for line in stream_reader:
        await yield_(json.loads(line))


  • decorate your function with @async_generator
  • replace yield with await yield_()
  • replace yield X with await yield_(X)

That’s it!

Yield from

Native async generators don’t support yield from:

# Doesn't work!
async def wrap_load_json_lines(stream_reader):
    # This is a SyntaxError
    yield from load_json_lines(stream_reader)

But we do:

from async_generator import async_generator, yield_from_

# This works!
async def wrap_load_json_lines(stream_reader):
    await yield_from_(load_json_lines(stream_reader))

You can only use yield_from_ inside an @async_generator function, BUT the thing you PASS to yield_from_ can be any kind of async iterator, including native async generators.

Our yield_from_ fully supports the classic yield from semantics, including forwarding asend and athrow calls into the delegated async generator, and returning values:

from async_generator import async_generator, yield_, yield_from_

async def agen1():
    await yield_(1)
    await yield_(2)
    return "great!"

async def agen2():
    value = await yield_from_(agen1())
    assert value == "great!"


For introspection purposes, we also export the following functions:


Returns true if passed either an async generator object created by this library, or a native Python 3.6+ async generator object. Analogous to inspect.isasyncgen() in 3.6+.


Returns true if passed either an async generator function created by this library, or a native Python 3.6+ async generator function. Analogous to inspect.isasyncgenfunction() in 3.6+.


>>> isasyncgenfunction(load_json_lines)
>>> gen_object = load_json_lines(asyncio_stream_reader)
>>> isasyncgen(gen_object)

In addition, this library’s async generator objects are registered with the collections.abc.AsyncGenerator abstract base class (if available):

>>> isinstance(gen_object, collections.abc.AsyncGenerator)


This library generally tries hard to match the semantics of Python 3.6’s native async generators in every detail (PEP 525), except that it adds yield from support, and it doesn’t currently support the sys.{get,set}_asyncgen_hooks garbage collection API. There are two main reasons for this: (a) it doesn’t exist on Python 3.5, and (b) even on 3.6, only built-in generators are supposed to use that API, and that’s not us. In any case, you probably shouldn’t be relying on garbage collection for async generators – see this discussion and PEP 533 for more details.

Context managers

As discussed above, you should always explicitly call aclose on async generators. To make this more convenient, this library also includes an aclosing async context manager. It acts just like the closing context manager included in the stdlib contextlib module, but does await obj.aclose() instead of obj.close(). Use it like this:

from async_generator import aclosing

async with aclosing(load_json_lines(asyncio_stream_reader)) as agen:
    async for json_obj in agen:

Or if you want to write your own async context managers, we got you covered:


This is a backport of contextlib.asynccontextmanager(), which wasn’t added to the standard library until Python 3.7.

You can use @asynccontextmanager with either native async generators, or the ones from this package. If you use it with the ones from this package, remember that @asynccontextmanager goes on top of @async_generator:

# Correct!
async def my_async_context_manager():

# This won't work :-(
async def my_async_context_manager():