Write a program in asyncio, that calls into a library that knows nothing about asyncio, which then calls out to a database adapter that is only asyncio

async_to_greenlet_to_async.py

"""Proof of concept of an adapter that will wrap a blocking IO library in
greenlets, so that it can be invoked by asyncio code and itself make calls out
to an asyncio database library.

hint:  any Python ORM or database abstraction tool written against the DBAPI
could in theory allow asyncio round trips to a pure async DB driver like
asyncpg.

The approach here seems too simple to be true and I did not expect it to
collapse down to something this minimal, so it is very possible I am totally
missing something here.   it's 1 AM and maybe I'm staring at it for too long
but it works?

note this is not using gevent or eventlet, we don't actually need those here
as we are using regular asyncio networking.   It uses only the underlying
greenlet library which allows us to context switch without the
"await" keyword or other asyncio syntax.

"""
import asyncio
import contextvars
import sys

import asyncpg
import greenlet


class AsyncConnection:
    """An asyncio database connection class.

    This is a pure asyncio class that has to be run in an
    explicit async event loop.

    Uses asyncpg.

    """

    @classmethod
    async def create(self, **kw):
        conn = AsyncConnection.__new__(AsyncConnection)
        conn.conn = await asyncpg.connect(**kw)
        return conn

    async def execute(self, statement):
        await self.conn.execute(statement)

    async def fetch(self, statement):
        return await self.conn.fetch(statement)

    async def close(self):
        await self.conn.close()


class SyncConnection:
    """A synchronous database connection class.

    This class mirrors the asyncpg API above however does not use asyncio at
    all, all methods are written in blocking style.

    This class is fairly analogous to the Connection in SQLAlchemy or other
    ORMs.  For a more DBAPI/SQLAlcheny-like approach we'd need to make more of
    a "cursor" effect from asyncpg and make sure it starts a transaction, etc.
    but in general this would be the SQLAlchemy Connection class, or the ORM
    Session (which in SQLAlchemy 2.0 is heavily based on Session.execute()).

    """
    @classmethod
    def create(self, impl, **kw):
        conn = SyncConnection.__new__(SyncConnection)
        conn.conn = impl.create(**kw)
        return conn

    def execute(self, statement):
        self.conn.execute(statement)

    def fetch(self, statement):
        return self.conn.fetch(statement)

    def close(self):
        self.conn.close()


class SyncAdaptImpl:
    """An adapter that translates between the async and sync versions
    of the Connection.

    This class can likely be generated programmatically for a larger
    scale application as this is basically "add the async keyword to
    each function call" boilerplate.

    """

    @classmethod
    def create(cls, **kw):
        conn = SyncAdaptImpl.__new__(SyncAdaptImpl)

        async def async_create(**kw):
            return await AsyncConnection.create(
                user="scott",
                password="tiger",
                host="localhost",
                database="test",
            )

        conn.async_conn = greenlet_runner.run_in_asyncio(async_create(**kw))
        return conn

    def execute(self, statement):
        async def execute(statement):
            await self.async_conn.execute(statement)

        return greenlet_runner.run_in_asyncio(execute(statement))

    def fetch(self, statement):
        async def fetch(statement):
            return await self.async_conn.fetch(statement)

        return greenlet_runner.run_in_asyncio(fetch(statement))

    def close(self):
        async def close():
            await self.async_conn.close()

        return greenlet_runner.run_in_asyncio(close())


def im_a_greenlet():
    """our greenlet-style function.

    All the code here is written against the SyncConnection in blocking style
    and this code knows nothing about asyncio.  It is written for blocking
    database connections, however we will run it in a greenlet container that
    allows it to talk to asyncio database connections.

    This would apply towards all the major chunks of the ORM that would
    be insane to rewrite just to have "async" keywords everywhere, namely
    the logic used by the unit of work to flush objects, and possibly
    code like eager loaders as that run inline with loading as well.

    """
    conn = SyncConnection.create(
        SyncAdaptImpl,
        user="scott",
        password="tiger",
        host="localhost",
        database="test",
    )

    conn.execute("drop table if exists mytable")

    conn.execute(
        "create table if not exists "
        "mytable (id serial primary key, data varchar)"
    )

    conn.execute("insert into mytable(data) values ('even more data')")

    result = conn.fetch("select id, data from mytable")

    conn.close()

    return result


class GreenletExecutor:
    """Greenlet exector that wires up greenlet.switch() and await calls
    together.

    I'm sort of not really sure why this even works and I have not tested
    this under real concurrency, this seems to simple to be true actually,
    I first wrote this using a more complicated approach with two separate
    event loops, but I'd prefer to have things run as a straight shot.

    If this effect is actually this simple and there's no spooky weird
    thing that is going to happen from adding stackless Python context
    switches into async / await, this would be kind of amazing?

    """
    current_return = contextvars.ContextVar("current greenlet context")

    def run_in_asyncio(self, coroutine):
        return self.current_return.get().switch(coroutine)

    async def run_greenlet(self, fn):

        result_future = asyncio.Future()

        def run_greenlet_target():
            result_future.set_result(fn())
            return None

        async def run_greenlet():
            gl = greenlet.greenlet(run_greenlet_target)
            greenlet_coroutine = gl.switch()

            while greenlet_coroutine is not None:
                task = asyncio.create_task(greenlet_coroutine)
                try:
                    await task
                except:
                    # this allows an exception to be raised within
                    # the moderated greenlet so that it can continue
                    # its expected flow.
                    greenlet_coroutine = gl.throw(*sys.exc_info())
                else:
                    greenlet_coroutine = gl.switch(task.result())

        current_return = greenlet.greenlet(run_greenlet)
        self.current_return.set(current_return)
        await current_return.switch()

        return result_future.result()


greenlet_runner = GreenletExecutor()


if __name__ == "__main__":
    # finally here's our program, written as asyncio.   we want to use
    # asyncio to write our app, but be able to call into a library
    # that knows nothing about asyncio, so we will adapt it to run
    # as a greenlet.   the trick is that the database library ultimately
    # used is also asyncio, so we don't need gevent or eventlet networking
    # or even their event loops, only the greenlet construct.

    loop = asyncio.get_event_loop()

    async def run_our_asyncio_program():
        retval = await greenlet_runner.run_greenlet(im_a_greenlet)
        print(retval)

    loop.run_until_complete(run_our_asyncio_program())

oh that's a great link, I have a vague notion that Guido has rejected all the very reasonable approaches such that asyncio doesnt force all library authors everywhere to rewrite everything fron scratch, but this seems to be where it actually happened.

re: microlibrary, I think at the moment this would be a utility wihtin SQLAlchemy itself but I don't have an appetite to maintain /release a separate package. Also considering "greenback" already does this I wouldn't want to compete with that.

that said, consider this code to be public domain and go nuts with it.