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https://github.com/oremanj/greenback

Reenter an asyncio or Trio event loop from synchronous code
https://github.com/oremanj/greenback

Last synced: 26 days ago
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Reenter an asyncio or Trio event loop from synchronous code

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greenback: reenter an asyncio or Trio event loop from synchronous code

======================================================================



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Python 3.5 introduced ``async``/``await`` syntax for defining

functions that can run concurrently in a cooperative multitasking

framework such as ``asyncio`` or `Trio

`__. Such frameworks have a number of advantages

over previous approaches to concurrency: they scale better than threads and are

`clearer about control flow `__

than the implicit cooperative multitasking provided by ``gevent``. They're also being

actively developed to explore some `new ideas about concurrent programming

`__.



Porting an existing codebase to ``async``/``await`` syntax can be

challenging, though, since it's somewhat "viral": only an async

function can call another async function. That means you don't just have

to modify the functions that actually perform I/O; you also need to

(trivially) modify every function that directly or indirectly calls a

function that performs I/O. While the results are generally an improvement

("explicit is better than implicit"), getting there in one big step is not

always feasible, especially if some of these layers are in libraries that

you don't control.



``greenback`` is a small library that attempts to bridge this gap. It

allows you to **call back into async code from a syntactically

synchronous function**, as long as the synchronous function was

originally called from an async task (running in an asyncio or Trio

event loop) that set up a ``greenback`` "portal" as explained

below. This is potentially useful in a number of different situations:



* You can interoperate with some existing libraries that are not

  ``async``/``await`` aware, without pushing their work off into

  another thread.



* You can migrate an existing program to ``async``/``await``

  syntax one layer at a time, instead of all at once.



* You can (cautiously) design async APIs that block in places where

  you can't write ``await``, such as on attribute accesses.



``greenback`` requires Python 3.8 or later and an implementation that

supports the ``greenlet`` library. Either CPython or PyPy should work.

There are no known OS dependencies.



Quickstart

----------



* Call ``await greenback.ensure_portal()`` at least once in each task that will be

  using ``greenback``. (Additional calls in the same task do nothing.) You can think

  of this as creating a portal that will be used by future calls to

  ``greenback.await_()`` in the same task.



* Later, use ``greenback.await_(foo())`` as a replacement for

  ``await foo()`` in places where you can't write ``await``.



* If all of the places where you want to use

  ``greenback.await_()`` are indirectly within a single function, you can

  eschew the ``await greenback.ensure_portal()`` and instead write a wrapper

  around calls to that function: ``await greenback.with_portal_run(...)``

  for an async function, or ``await greenback.with_portal_run_sync(...)``

  for a synchronous function. These have the advantage of cleaning up the

  portal (and its associated minor performance impact) as soon as the

  function returns, rather than leaving it open until the task terminates.



* For more details and additional helper methods, see the

  `documentation `__.



Example

-------



Suppose you start with this async-unaware program:



.. code-block:: python



    import subprocess



    def main():

        print_fact(10)



    def print_fact(n, mult=1):

        """Print the value of *n* factorial times *mult*."""

        if n <= 1:

            print_value(mult)

        else:

            print_fact(n - 1, mult * n)



    def print_value(n):

        """Print the value *n* in an unreasonably convoluted way."""

        assert isinstance(n, int)

        subprocess.run(f"echo {n}", shell=True)



    if __name__ == "__main__":

        main()



Using ``greenback``, you can change it to run in a Trio event loop by

changing only the top and bottom layers, with no change to ``print_fact()``.



.. code-block:: python



    import trio

    import greenback



    async def main():

        await greenback.ensure_portal()

        print_fact(10)



    def print_fact(n, mult=1):

        """Print the value of *n* factorial times *mult*."""

        if n <= 1:

            print_value(mult)

        else:

            print_fact(n - 1, mult * n)



    def print_value(n):

        """Print the value *n* in an unreasonably convoluted way."""

        assert isinstance(n, int)

        greenback.await_(trio.run_process(f"echo {n}", shell=True))



    if __name__ == "__main__":

        trio.run(main)



FAQ

---



**Why is it called "greenback"?** It uses the `greenlet

`__ library to get you

*back* to an enclosing async context. Also, maybe it saves you `money

`__ (engineering time) or

something.



**How does it work?** After you run ``await greenback.ensure_portal()``

in a certain task, that task will run inside a greenlet.

(This is achieved by interposing a "shim" coroutine in between the event

loop and the coroutine for your task; see the source code for details.)

Calls to ``greenback.await_()`` are then able to switch from that greenlet

back to the parent greenlet, which can easily perform the necessary

``await`` since it has direct access to the async environment. The

task greenlet is then resumed with the value or exception

produced by the ``await``.



**Should I trust this in production?** Maybe; try it and see. The

technique is rather low-level, and has some minor

`performance implications `__ (any task in which you call

``await greenback.ensure_portal()`` will run a bit slower), but we're in

good company: SQLAlchemy's async ORM support is implemented in much

the same way.  ``greenback`` itself is a fairly small amount of

pure-Python code on top of ``greenlet``. (There is one small usage of

``ctypes`` to work around a knob that's not exposed by the asyncio

acceleration extension module on CPython.)

``greenlet`` is a C module full of platform-specific arcana, but

it's been around for a very long time and popular production-quality

concurrency systems such as ``gevent`` rely heavily on it.



**What won't work?** A few things:



* Greenlet switching works by moving parts of the C stack to different

  memory addresses, relying on the assumption that Python objects are

  fully heap-allocated and don't contain any pointers into the C

  stack. Poorly-behaved C extension modules might violate this

  assumption and are likely to crash if used with ``greenback``.

  Such extension modules are buggy and could be made to crash without

  ``greenback`` too, but perhaps only under an obscure or unlikely

  series of operations.



* Calling ``greenback.await_()`` inside a finalizer (``__del__``

  method), signal handler, or weakref callback is unsupported. It

  might work most of the time, or even all the time, but the

  environment in which such methods run is weird enough that the

  author isn't prepared to make any guarantees.  (Not that you have

  any guarantees about the rest of it, just some better theoretical

  grounding.)



License

-------



``greenback`` is licensed under your choice of the MIT or Apache 2.0 license.

See `LICENSE `__

for details.