Mastering asyncio

What’s asyncio?

asyncio is a Python 3’s built-in library. This means it’s already installed if you have Python 3. Since Python 3.5, it is convenient to work with asynchronous code. Before (Python 3.4) we didn’t have async or await, but now we do.

asyncio stands for Asynchronous Input Output. This is a very powerful concept to use whenever you work IO. Interacting with the web or external APIs such as Telegram’s makes a lot of sense this way.

Why asyncio?

Asynchronous IO makes a lot of sense in a library like Telethon. You send a request to the server (such as “get some message”), and thanks to asyncio, your code won’t block while a response arrives.

The alternative would be to spawn a thread for each update so that other code can run while the response arrives. That is a lot more expensive.

The code will also run faster, because instead of switching back and forth between the OS and your script, your script can handle it all. Avoiding switching saves quite a bit of time, in Python or any other language that supports asynchronous IO. It will also be cheaper, because tasks are smaller than threads, which are smaller than processes.

What are asyncio basics?

The code samples below assume that you have Python 3.7 or greater installed.

# First we need the asyncio library
import asyncio

# We also need something to run
async def main():
    for char in 'Hello, world!\n':
        print(char, end='', flush=True)
        await asyncio.sleep(0.2)

# Then, we can create a new asyncio loop and use it to run our coroutine.
# The creation and tear-down of the loop is hidden away from us.

What does telethon.sync do?

The moment you import any of these:

from telethon import sync, ...
# or
from telethon.sync import ...
# or
import telethon.sync

The sync module rewrites most async def methods in Telethon to something similar to this:

def new_method():
    result = original_method()
    if loop.is_running():
        # the loop is already running, return the await-able to the user
        return result
        # the loop is not running yet, so we can run it for the user
        return loop.run_until_complete(result)

That means you can do this:


Instead of this:

me = client.loop.run_until_complete(client.get_me())

# or, using asyncio's default loop (it's the same)
import asyncio
loop = asyncio.get_running_loop()  # == client.loop
me = loop.run_until_complete(client.get_me())

As you can see, it’s a lot of boilerplate and noise having to type run_until_complete all the time, so you can let the magic module to rewrite it for you. But notice the comment above: it won’t run the loop if it’s already running, because it can’t. That means this:

async def main():
    # 3. the loop is running here
        client.get_me()  # 4. this will return a coroutine!
        .username  # 5. this fails, coroutines don't have usernames

loop.run_until_complete(  # 2. run the loop and the ``main()`` coroutine
    main()  # 1. calling ``async def`` "returns" a coroutine

Will fail. So if you’re inside an async def, then the loop is running, and if the loop is running, you must await things yourself:

async def main():
    print((await client.get_me()).username)


What are async, await and coroutines?

The async keyword lets you define asynchronous functions, also known as coroutines, and also iterate over asynchronous loops or use async with:

import asyncio

async def main():
    # ^ this declares the main() coroutine function

    async with client:
        # ^ this is an asynchronous with block

        async for message in client.iter_messages(chat):
            # ^ this is a for loop over an asynchronous generator

# ^ this will create a new asyncio loop behind the scenes and tear it down
#   once the function returns. It will run the loop untiil main finishes.
#   You should only use this function if there is no other loop running.

The await keyword blocks the current task, and the loop can run other tasks. Tasks can be thought of as “threads”, since many can run concurrently:

import asyncio

async def hello(delay):
    await asyncio.sleep(delay)  # await tells the loop this task is "busy"
    print('hello')  # eventually the loop resumes the code here

async def world(delay):
    # the loop decides this method should run first
    await asyncio.sleep(delay)  # await tells the loop this task is "busy"
    print('world')  # eventually the loop finishes all tasks

async def main():
    asyncio.create_task(world(2))  # create the world task, passing 2 as delay
    asyncio.create_task(hello(delay=1))  # another task, but with delay 1
    await asyncio.sleep(3)  # wait for three seconds before exiting

    # create a new temporary asyncio loop and use it to run main
except KeyboardInterrupt:

The same example, but without the comment noise:

import asyncio

async def hello(delay):
    await asyncio.sleep(delay)

async def world(delay):
    await asyncio.sleep(delay)

async def main():
    await asyncio.sleep(3)

except KeyboardInterrupt:

Can I use threads?

Yes, you can, but you must understand that the loops themselves are not thread safe. and you must be sure to know what is happening. The easiest and cleanest option is to use to create and manage the new event loop for you:

import asyncio
import threading

async def actual_work():
    client = TelegramClient(..., loop=loop)
    ...  # can use `await` here

def go():


Generally, you don’t need threads unless you know what you’re doing. Just create another task, as shown above. If you’re using the Telethon with a library that uses threads, you must be careful to use threading.Lock whenever you use the client, or enable the compatible mode. For that, see Compatibility and Convenience.

You may have seen this error:

RuntimeError: There is no current event loop in thread 'Thread-1'.

It just means you didn’t create a loop for that thread. Please refer to the asyncio documentation to correctly learn how to set the event loop for non-main threads.

client.run_until_disconnected() blocks!

All of what client.run_until_disconnected() does is run the asyncio’s event loop until the client is disconnected. That means the loop is running. And if the loop is running, it will run all the tasks in it. So if you want to run other code, create tasks for it:

from datetime import datetime

async def clock():
    while True:
        print('The time:',
        await asyncio.sleep(1)


This creates a task for a clock that prints the time every second. You don’t need to use client.run_until_disconnected() either! You just need to make the loop is running, somehow. loop.run_forever() and loop.run_until_complete() can also be used to run the loop, and Telethon will be happy with any approach.

Of course, there are better tools to run code hourly or daily, see below.

What else can asyncio do?

Asynchronous IO is a really powerful tool, as we’ve seen. There are plenty of other useful libraries that also use asyncio and that you can integrate with Telethon.

  • aiohttp is like the infamous requests but asynchronous.
  • quart is an asynchronous alternative to Flask.
  • aiocron lets you schedule things to run things at a desired time, or run some tasks hourly, daily, etc.

And of course, asyncio itself! It has a lot of methods that let you do nice things. For example, you can run requests in parallel:

async def main():
    last, sent, download_path = await asyncio.gather(
        client.get_messages('telegram', 10),
        client.send_message('me', 'Using asyncio!'),


This code will get the 10 last messages from @telegram, send one to the chat with yourself, and also download the profile photo of the channel. asyncio will run all these three tasks at the same time. You can run all the tasks you want this way.

A different way would be:

loop.create_task(client.get_messages('telegram', 10))
loop.create_task(client.send_message('me', 'Using asyncio!'))

They will run in the background as long as the loop is running too.

You can also start an asyncio server in the main script, and from another script, connect to it to achieve Inter-Process Communication. You can get as creative as you want. You can program anything you want. When you use a library, you’re not limited to use only its methods. You can combine all the libraries you want. People seem to forget this simple fact!

Why does client.start() work outside async?

Because it’s so common that it’s really convenient to offer said functionality by default. This means you can set up all your event handlers and start the client without worrying about loops at all.

Using the client in a with block, start, run_until_disconnected, and disconnect all support this.

Where can I read more?

Check out my blog post about asyncio, which has some more examples and pictures to help you understand what happens when the loop runs.