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.

pip3 install nest-asyncio2

Python 3.5 or higher is required.

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.

# /// 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())

If there is no existing event loop, patched asyncio.run() will create one but not close it afterwards. It will be reused later, so there will be at most one leaked loop.

asyncio.run() will always create and close the loop. But nest_asyncio (by accident or intentionally) missed it. As changing this behavior will break existing projects (e.g. ComfyScript, pyvista), nest-asyncio2 follows this behavior.

This will cause a ResourceWarning: unclosed event loop at exit on Python 3.12+, although it is hidden by default. (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.)

If you want to follow asyncio.run()'s behavior and get rid of the ResourceWarning, you can set run_close_loop=True for all apply():

nest_asyncio2.apply(run_close_loop=True)

Or pass loop_factory to asyncio.run() on Python 3.12+:

asyncio.run(..., loop_factory=asyncio.new_event_loop)

nest-asyncio2 v2 may change run_close_loop to be enabled by default.

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

• Support setting run_close_loop to avoid leaked event loop.
• Python 3.12 support
  • loop_factory parameter support

• 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

• To avoid potential bugs, apply() will warn if asyncio is already patched by nest_asyncio on Python 3.12+.

  You can also set error_on_mispatched=True to turn the warning into a RuntimeError, regardless of the Python version. See #1 for details.

There is no behavior change on Python < 3.12.

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