.. _mastering-asyncio: ================= Mastering asyncio ================= .. contents:: 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. .. code-block:: python # 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. asyncio.run(main()) What does telethon.sync do? =========================== The moment you import any of these: .. code-block:: python 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: .. code-block:: python def new_method(): result = original_method() if loop.is_running(): # the loop is already running, return the await-able to the user return result else: # 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: .. code-block:: python print(client.get_me().username) Instead of this: .. code-block:: python me = client.loop.run_until_complete(client.get_me()) print(me.username) # 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()) print(me.username) 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: .. code-block:: python async def main(): # 3. the loop is running here print( 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: .. code-block:: python async def main(): print((await client.get_me()).username) loop.run_until_complete(main()) 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``: .. code-block:: python 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 print(message.sender.username) asyncio.run(main()) # ^ 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: .. code-block:: python 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 try: # create a new temporary asyncio loop and use it to run main asyncio.run(main()) except KeyboardInterrupt: pass The same example, but without the comment noise: .. code-block:: python import asyncio async def hello(delay): await asyncio.sleep(delay) print('hello') async def world(delay): await asyncio.sleep(delay) print('world') async def main(): asyncio.create_task(world(2)) asyncio.create_task(hello(delay=1)) await asyncio.sleep(3) try: asyncio.run(main()) except KeyboardInterrupt: pass 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 `asyncio.run` to create and manage the new event loop for you: .. code-block:: python import asyncio import threading async def actual_work(): client = TelegramClient(..., loop=loop) ... # can use `await` here def go(): asyncio.run(actual_work()) threading.Thread(target=go).start() 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 :ref:`compatibility-and-convenience`. You may have seen this error: .. code-block:: text 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: .. code-block:: python from datetime import datetime async def clock(): while True: print('The time:', datetime.now()) await asyncio.sleep(1) loop.create_task(clock()) ... client.run_until_disconnected() 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: .. code-block:: python async def main(): last, sent, download_path = await asyncio.gather( client.get_messages('telegram', 10), client.send_message('me', 'Using asyncio!'), client.download_profile_photo('telegram') ) loop.run_until_complete(main()) 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: .. code-block:: python loop.create_task(client.get_messages('telegram', 10)) loop.create_task(client.send_message('me', 'Using asyncio!')) loop.create_task(client.download_profile_photo('telegram')) 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.