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

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 exciting 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 async 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 helpers, read the rest of this documentation!

Detailed documentation

Indices and tables