Combining Traditional Thread-Based Code and Asyncio in Python

A comprehensive guide to integrating synchronous and asynchronous programming in Python

Combining Traditional Thread-Based Code and Asyncio in Python
Photo Credit: Created by Author, Canva

In this article, I’ll explain how to call existing IO-blocking code in asyncio programs that don’t implement asyncio and how to call asyncio code in existing programs based on the threaded model.


In the previous articles, I introduced you to asyncio, a Python feature. The performance of asyncio is very high, and using asyncio in modern, highly concurrent code will improve IO performance by several orders of magnitude.

But in the real world, we have not seen asyncio code used as much as expected. Why is that?

Challenge 1: How to call old IO-blocking code in asyncio code

One scenario is that while we implement the new code with asyncio, there are still a lot of IO-blocking programs left in the system that was implemented traditionally.

For example, microservice, file operation, etc. Even if you use asyncio and call these blocking APIs directly, you still can’t achieve the high concurrency effect.

Challenge 2: How to call asyncio in existing blocking code to accomplish tasks asynchronously

In another case, the existing code already implements a set of architecture based on the threaded model.

Since asyncio’s event loop is executed in the current thread, directly calling asyncio will block the execution of existing code. It does not have the effect of concurrent execution.

So today, I will use a few real-life examples to show you how to implement asyncio calls in each of the two cases.

Part 1: Calling IO-Blocking Code in Asyncio-Based Programs

Let’s take FastAPI as an example. FastAPI is a high-performance web framework based on asyncio implementation.

But very often, not all the business logic of a web application is implemented in the FastAPI code.

Sometimes we need to call several microservices implemented long ago that are blocking calls. How could we deal with this situation?

Using run_in_executor to Run IO-Blocking Code

In the previous article, we explained how to use loop.run_in_executor API to integrate multiple processes with asyncio to achieve high-performance computing.

Combining Multiprocessing and Asyncio in Python for Performance Boosts
Combining Multiprocessing and asyncio via run_in_executor unifies the API for concurrent and parallel programming, simplifies our programming process, and allows us to obtain execution results in order of completion. This article will use a Real-world Example to Explain the Code Implementation

However, IO-bound code is unsuitable for multi-process calls but recommended for multi-thread.

The good thing is that the first argument of loop.run_in_executor accepts either a concurrent.futures.ProcessPoolExecutor implementation or a concurrent.futures.ThreadPoolExecutor implementation. So our sample code is as follows:

import asyncio
from concurrent.futures import ThreadPoolExecutor
from typing import Callable, Any

from fastapi import FastAPI
import requests

app = FastAPI()

def get_status(q: str):
    """ Use the requests package to simulate microservice calls """
    response = requests.get(f"{q}")
    return response.status_code

async def run_in_thread(fn: Callable, *args) -> Any:
    """ A tool method is used to call loop.run_in_executor """
    task = app.state._loop.run_in_executor(
        app.state.executor, fn, *args
    return await task

async def info(q: str):
    """ Calling synchronous methods within asynchronous methods """
    status_code = await run_in_thread(get_status, (q,))
    return f"The status is {status_code}"

async def on_startup():
    """ Initialize the thread pool and event loop at program startup """
    app.state.executor = ThreadPoolExecutor()
    app.state._loop = asyncio.get_running_loop()

async def on_shutdown():
    """ Recycle resources when the program is shut down """

First, we use a get_status method to simulate the old microservice code calls via the requests package.

Then, we manage the creation and destruction of the ThreadPoolExecutor thread pool during the startup and shutdown phases of the web application, respectively.

Finally, we call the IO blocking methods in the thread pool and get the results in the response method of the request via loop.run_in_executor.

The default executor parameter of loop.run_in_executor can be None. That is because asyncio will initialize a default thread pool internally after startup.

When the executor parameter of run_in_executor is None, it will use the default thread pool to execute, so we don’t have to manage a thread pool in our code:

async def run_in_thread(fn: Callable, *args) -> Any:
    """ A tool method is used to call loop.run_in_executor """
    task = app.state._loop.run_in_executor(
        None, fn, *args
    return await task

Leveraging asyncio.to_thread (Python 3.9+)

Python 3.9 introduced a new high-level abstraction API, asyncio.to_thread, which, as you can see from the source code, internally calls the loop.run_in_thread method with the executor argument of None:

async def info(q: str):
    """ Calling synchronous methods within asynchronous methods """
    status_code = await asyncio.to_thread(get_status, (q,))
    return f"The status is {status_code}"

Thus, using asyncio.to_thread, will further simplify the code.

Part 2: Calling Asyncio Code in Traditional Thread-Based Programs

There is another case where our program already implements a loop in the existing code.

For example, most GUI programs use an event loop to respond to various events and to update the UI.

Let’s take tkinter as an example. tkinter will start a main loop when it starts, and this main loop will block the main thread and keep on looping. As shown in the figure below:

How does the tkinter main loop work.
How does the tkinter main loop work. Image by Author

A direct call to synchronous IO code will block the main loop

Let’s take the example of a tkinter program that contains a button and a status text:

import tkinter as tk

import requests

class App(tk.Tk):
    INIT_STATE = 0         # Initial state
    QUERYING_STATE = 1     # State when an HTTP request is started
    RESULT_STATE = 2       # The state of the HTTP request result

    def __init__(self):
        self.status_code = 0
        self._refresh_ms = 60
        self.state = App.INIT_STATE
        self._button = None
        self._label = None
        self.after(self._refresh_ms, self.refresh)

    def render_elements(self):
        """ Set up the canvas and render UI elements """
        self._button = tk.Button(self, text="request code", command=self.request_remote)
        self._label = tk.Label(self, text="")


    def request_remote(self):
        """ Initiate HTTP requests in a synchronous manner """
        self.state = App.QUERYING_STATE
        response = requests.get("")
        self.status_code = response.status_code
        self.state = App.RESULT_STATE

    def refresh(self):
        """ Refresh UI content every 60 milliseconds """
        self.after(self._refresh_ms, self.refresh)

    def update_label(self):
        """ Update text content according to APP state """
        match self.state:
            case App.INIT_STATE:
                self._label.config(text="Here will show the status code.")
            case App.QUERYING_STATE:
                self._label.config(text="Query remote...")
            case App.RESULT_STATE:
                self._label.config(text=f"The result code is: {self.status_code}")

    def start(self):

def main():
    app = App()

if __name__ == "__main__":

This program uses a state machine to implement it. Every 60 milliseconds, the code refreshes the corresponding text according to the program’s current state.

When we click the request_code button, the workflow should ideally look like the following diagram:

Workflow of the tkinter program.
Workflow of the tkinter program. Image by Author

But by the execution result, the program hangs when clicking the button, and the status text is updated until the IO blocking code finishes executing.

It means that the main loop is blocked when the IO request is running, causing the GUI interface to be unresponsive:

The app is blocked and doesn’t show query text.
The app is blocked and doesn’t show query text. Image by Author

Using to Run Asyncio Code

Can we replace the requests package with the aiohttp package to achieve the asynchronous invocation of IO requests?

Here we first inherit the App class to implement a new class AppAsyncBase. In this new class, we use aiohttp to implement an async_request method to lay the foundation for subsequent asynchronous calls:

class AppAsyncBase(App):
    async def async_request(self):
        """ Asynchronous initiation of HTTP calls using aiohttp """
        async with aiohttp.ClientSession() as session:
            async with session.get("") as response:
                self.status_code = response.status
                self.state = App.RESULT_STATE

Readers of my previous article will know we can execute asynchronous methods inside synchronous code via

Harnessing Multi-Core Power with Asyncio in Python
In this article, I will show you how to execute Python asyncio code on a multi-core CPU to unlock the full performance of concurrent tasks.

Then, we implement a new class AppAsyncRun, by inheriting AppAsyncBase.

In this new class, we override the request_remote method and use to call the async_request method directly:

class AppAsyncRun(AppAsyncBase):
    def __int__(self):

    def request_remote(self):
        """ Use to call the concurrent code """
        self.state = AppAsyncBase.QUERYING_STATE

Next, let’s look at the results. Because asyncio’s event loop is executed in the main thread by default, and when the event loop is running, it blocks the main thread, and the main loop of tkinter is blocked and unresponsive: blocks the main loop. blocks the main loop. Image by Author

Integrating Asyncio with Thread-Based Programs

Is there a way to solve the event loop blocking problem?

Here we can use a separate daemon thread and then run the event loop into the daemon thread, so asyncio’s event loop will not block the main thread. The diagram is as follows:

Combining tkinter and asyncio loops.
Combining tkinter and asyncio loops. Image by Author

Looking at the code implementation, we first inherit the AppAsyncBase class to implement a new class AppEventLoop.

Next, override the request_remote method and use asyncio.run_coroutine_threadsafe to call the async_request method in the event loop.

Request method in the event loop. asyncio.run_coroutine_threadsafe is also thread-safe:

class AppEventLoop(AppAsyncBase):
    def __init__(self, loop: AbstractEventLoop = None):
        self._loop = loop

    def request_remote(self):
        """ Run the coroutine in event loop """
        self.state = AppAsyncBase.QUERYING_STATE
        asyncio.run_coroutine_threadsafe(self.async_request(), self._loop)

Implement a run_event_loop method to call the loop.run_forever in the thread:

def run_event_loop(loop: AbstractEventLoop):
    """ Run asyncio event loop in daemon thread """

Then, use the contextmanager decorator to manage the lifecycle of the daemon thread:

def get_event_loop():
    """ Use a context manager to manage the thread's lifecycle"""
    loop = asyncio.get_event_loop()
    thread = Thread(target=run_event_loop, args=(loop,))
    thread.daemon = True
        yield loop

Finally, implement the event loop integration and the app launch in the main method, and let’s see the result:

def main():
    with get_event_loop() as loop:
        app = AppEventLoop(loop)
The event loop running separately in the daemon thread no longer blocks.
The event loop running separately in the daemon thread no longer blocks. Image by Author

Perfect! Click the button, the status text is changed accordingly, the whole GUI interface runs smoothly, and IO calls do not block the GUI ever. Mission accomplished.


Although asyncio can dramatically improve the execution performance of concurrent programs, asyncio is not used on a large scale because it does not implement much of the legacy code.

Today’s article, using examples from real-world coding efforts, demonstrates the solution to two challenges:

  1. How to call the old IO code non-blocking in a new asyncio program.
  2. How to use asyncio asynchronous code in an existing synchronous program to achieve non-blocking execution.

Welcome to leave your comments and discussions. I will answer them one by one.

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