nest-asyncio2 1.6.2

Patch asyncio to allow nested event loops

nest-asyncio2

Introduction

By design asyncio does not allow its event loop to be nested. This presents a practical problem: When in an environment where the event loop is already running it's impossible to run tasks and wait for the result. Trying to do so will give the error "RuntimeError: This event loop is already running".

The issue pops up in various environments, such as web servers, GUI applications and in Jupyter notebooks.

This module patches asyncio to allow nested use of asyncio.run and loop.run_until_complete.

Installation

pip3 install nest-asyncio2

Python 3.5 or higher is required.

Usage

import nest_asyncio2
nest_asyncio2.apply()

Optionally the specific loop that needs patching can be given as argument to apply, otherwise the current event loop is used. An event loop can be patched whether it is already running or not. Only event loops from asyncio can be patched; Loops from other projects, such as uvloop or quamash, generally can't be patched.

Examples

aiohttp

# /// script
# requires-python = ">=3.5"
# dependencies = [
#     "aiohttp",
#     "nest-asyncio2",
# ]
# ///
import asyncio
import nest_asyncio2
import aiohttp

nest_asyncio2.apply()

async def f_async():
    # Note that ClientSession must be created and used
    # in the same event loop (under the same asyncio.run())
    async with aiohttp.ClientSession() as session:
        async with session.get('http://httpbin.org/get') as resp:
            print(resp.status)
            print(await resp.text())
            assert resp.status == 200

# async to sync
def f():
    asyncio.run(f_async())

async def main():
    f()
asyncio.run(main())

Comparison with nest_asyncio

nest-asyncio2 is a fork of the unmaintained nest_asyncio, with the following changes:

• Python 3.12 support

  • loop_factory parameter support

  • Fix ResourceWarning: unclosed event loop at exit

    Note that if you call asyncio.get_event_loop() on the main thread without setting the loop before, ResourceWarning is expected on Python 3.12~3.13, not caused by nest-asyncio2.

• Python 3.14 support
  • Fix broken asyncio.current_task() and others
  • Fix DeprecationWarning: 'asyncio.get_event_loop_policy' is deprecated and slated for removal in Python 3.16

All interfaces are kept as they are. To migrate, you just need to change the package and module name to nest_asyncio2.