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Description:

greenback 1.2.1

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:
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().
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.