greenback
Reenter an async event loop from synchronous code
Decision gist · record as of 2026-08-14
Yes, if you need to migrate existing sync code to async incrementally or interoperate with non-async libraries without threading. The library is actively maintained, has no known vulnerabilities, low install friction, and is permissively licensed. The main trade-off is a small performance cost per task and the requirement that C extensions used alongside greenback must be well-behaved. For greenfield async projects, it's unnecessary; for bridging sync and async, it's a solid choice.AI-flagged interpretation of the facts on this page — verify before relying
Before you install
- Requires Python 3.8+.
- Greenlet must be available; works on CPython and PyPy but not other implementations.
- Poorly-behaved C extensions may crash when used with greenback.
License · maintenance · safety
MIT -or- Apache License 2.0 (permissive) — Dual-licensed under MIT or Apache 2.0 (permissive). You may choose either license for your use, giving flexibility for both proprietary and open-source projects.
last release 2025-12-23 (234 days) · last repo commit 2026-02-17 · 92 stars
0 known vulnerabilities (OSV.dev, 2026-08-14) · 9,609,038 downloads/mo, #1,520 on PyPI
Alternatives
Verify before relying
import greenback
async def main():
await greenback.ensure_portal()
result = greenback.await_(some_async_function())
await main()- Whether the minor performance impact of running tasks inside a greenlet is acceptable for your specific workload.
- Compatibility with C extension modules in your dependency tree—greenback may expose latent bugs in extensions that violate greenlet's assumptions about heap allocation.
- Whether greenback.await_() works reliably with all async frameworks beyond asyncio and Trio.
What it is and what it does
Greenback solves the problem of incrementally migrating a synchronous codebase to async/await syntax. When you port code to asyncio or Trio, you typically must convert every function in the call chain to async, even if most of them don't perform I/O themselves. Greenback lets you call async functions from synchronous code, as long as that sync code was originally invoked from an async task that set up a greenback portal.
It works by running your async task inside a greenlet, then using greenlet's context-switching to jump back to the parent greenlet (which has direct access to the event loop) when you call greenback.await_(). The result or exception from the async call is then returned to the task greenlet. The library is pure Python on top of the mature greenlet C module, with minimal overhead.
Use it for
- Incrementally port a large codebase to async/await one layer at a time, leaving middle-layer functions unchanged.
- Interoperate with existing libraries that don't support async without spawning threads or rewriting their internals.
- Design async APIs that need to block in places where await syntax is forbidden, such as property getters.
- Call async functions from synchronous wrapper functions in a legacy codebase without full conversion.
- Bridge between synchronous and asynchronous code paths within the same event loop task.
Worth the install?
AI-flagged interpretation of the facts on this page. Verify before relying on it.
Yes, if you need to migrate existing sync code to async incrementally or interoperate with non-async libraries without threading.
The library is actively maintained, has no known vulnerabilities, low install friction, and is permissively licensed. The main trade-off is a small performance cost per task and the requirement that C extensions used alongside greenback must be well-behaved. For greenfield async projects, it's unnecessary; for bridging sync and async, it's a solid choice.
Install
greenback on PyPI
Before you install
Low friction: pure Python wheel with only three runtime dependencies (greenlet, sniffio, outcome). Actively maintained as of 2026-02-17 with 92 repository stars. Requires Python 3.8 or later and greenlet support (CPython and PyPy both work).
Requires Python 3.8+. Greenlet must be available; works on CPython and PyPy but not other implementations. Poorly-behaved C extensions may crash when used with greenback.
License in practice
Dual-licensed under MIT or Apache 2.0 (permissive). You may choose either license for your use, giving flexibility for both proprietary and open-source projects.
Quickstart
import greenback
async def main():
await greenback.ensure_portal()
result = greenback.await_(some_async_function())
await main()Verify before relying
- Whether the minor performance impact of running tasks inside a greenlet is acceptable for your specific workload.
- Compatibility with C extension modules in your dependency tree—greenback may expose latent bugs in extensions that violate greenlet's assumptions about heap allocation.
- Whether greenback.await_() works reliably with all async frameworks beyond asyncio and Trio.
Package facts
| License | MIT -or- Apache License 2.0 permissive |
| Python support | Supports the current Python release >=3.8 |
| Install friction | Low. Pure-Python wheel |
| Runtime dependencies | 3 packagesgreenletsniffiooutcome |
| Maintenance | Actively maintained 234 days since the last release |
| Last repo commit | |
| First released | |
| Downloads | 9,609,038 / month, #1,520 on PyPI 30-day window, as of 2026-08-14 |
| Known vulnerabilities | None known OSV.dev, checked 2026-08-14 |
| Classifiers | Development Status :: 4 - BetaFramework :: AsyncIOFramework :: TrioLicense :: OSI Approved :: Apache Software LicenseLicense :: OSI Approved :: MIT LicenseOperating System :: MacOS :: MacOS XOperating System :: Microsoft :: WindowsOperating System :: POSIX :: LinuxProgramming Language :: Python :: 3 :: OnlyProgramming Language :: Python :: Implementation :: CPythonProgramming Language :: Python :: Implementation :: PyPy |
Evidence: greenback-1.3.0-py3-none-any.whl
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See also greenlet · async_generator · duet · gevent · threadloop · asyncgui · sniffio · synchronicity · asyncer · culsans