The async_generator library

Python 3.6 added async generators. (What’s an async generator? Check out my 5-minute lightning talk demo from PyCon 2016.) Python 3.7 adds some more tools to make them usable, like contextlib.asynccontextmanager.

This library gives you all that back to Python 3.5.

For example, this code only works in Python 3.6+:

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

But this code does the same thing, and works on Python 3.5+:

from async_generator import async_generator, yield_

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

Or in Python 3.7, you can write:

from contextlib import asynccontextmanager

@asynccontextmanager
async def background_server():
    async with trio.open_nursery() as nursery:
        value = await nursery.start(my_server)
        try:
            yield value
        finally:
            # Kill the server when the scope exits
            nursery.cancel_scope.cancel()

This is the same, but back to 3.5:

from async_generator import async_generator, yield_, asynccontextmanager

@asynccontextmanager
@async_generator
async def background_server():
    async with trio.open_nursery() as nursery:
        value = await nursery.start(my_server)
        try:
            await yield_(value)
        finally:
            # Kill the server when the scope exits
            nursery.cancel_scope.cancel()

(And if you’re on 3.6, you can use @asynccontextmanager with native generators.)

Let’s do this

• Install: python3 -m pip install -U async_generator (or on Windows, maybe py -3 -m pip install -U async_generator
• Manual: https://async-generator.readthedocs.io/
• Bug tracker and source code: https://github.com/python-trio/async_generator
• Real-time chat: https://gitter.im/python-trio/general
• License: MIT or Apache 2, your choice
• Contributor guide: https://trio.readthedocs.io/en/latest/contributing.html
• Code of conduct: Contributors are requested to follow our code of conduct in all project spaces.

How come some of those links talk about “trio”?

Trio is a new async concurrency library for Python that’s obsessed with usability and correctness – we want to make it easy to get things right. The async_generator library is maintained by the Trio project as part of that mission, and because Trio uses async_generator internally.

You can use async_generator with any async library. It works great with asyncio, or Twisted, or whatever you like. (But we think Trio is pretty sweet.)