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Completely overhaul the documentation

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.. _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?
========================
.. code-block:: python
# First we need the asyncio library
import asyncio
# Then we need a loop to work with
loop = asyncio.get_event_loop()
# 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 need to run the loop with a task
loop.run_until_complete(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
import asyncio
loop = asyncio.get_event_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)
loop = asyncio.get_event_loop()
# ^ this assigns the default event loop from the main thread to a variable
loop.run_until_complete(main())
# ^ this runs the *entire* loop until the main() function finishes.
# While the main() function does not finish, the loop will be running.
# While the loop is running, you can't run it again.
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
loop = asyncio.get_event_loop() # get the default loop for the main thread
loop.create_task(world(2)) # create the world task, passing 2 as delay
loop.create_task(hello(delay=1)) # another task, but with delay 1
try:
# run the event loop forever; ctrl+c to stop it
# we could also run the loop for three seconds:
# loop.run_until_complete(asyncio.sleep(3))
loop.run_forever()
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')
loop = asyncio.get_event_loop()
loop.create_task(world(2))
loop.create_task(hello(1))
loop.run_until_complete(asyncio.sleep(3))
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. You
may want to create a loop in a new thread and make sure to pass it to
the client:
.. code-block:: python
import asyncio
import threading
def go():
loop = asyncio.new_event_loop()
client = TelegramClient(..., loop=loop)
...
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, and if you don't
pass a loop when creating the client, it uses ``asyncio.get_event_loop()``,
which only works in the main thread.
client.run_until_disconnected() blocks!
=======================================
All of what `client.run_until_disconnected()
<telethon.client.updates.UpdateMethods.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()
<telethon.client.updates.UpdateMethods.run_until_disconnected>` either!
You just need to make the loop is running, somehow. ``asyncio.run_forever``
and ``asyncio.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 <https://github.com/aio-libs/aiohttp>`_ is like the infamous
`requests <https://github.com/requests/requests/>`_ but asynchronous.
* `quart <https://gitlab.com/pgjones/quart>`_ is an asynchronous alternative
to `Flask <http://flask.pocoo.org/>`_.
* `aiocron <https://github.com/gawel/aiocron>`_ lets you schedule things
to run things at a desired time, or run some tasks hourly, daily, etc.
And of course, `asyncio <https://docs.python.org/3/library/asyncio.html>`_
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('TelethonChat', 10),
client.send_message('TelethonOfftopic', 'Hey guys!'),
client.download_profile_photo('TelethonChat')
)
loop.run_until_complete(main())
This code will get the 10 last messages from `@TelethonChat
<https://t.me/TelethonChat>`_, send one to `@TelethonOfftopic
<https://t.me/TelethonOfftopic>`_, and also download the profile
photo of the main group. 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('TelethonChat', 10))
loop.create_task(client.send_message('TelethonOfftopic', 'Hey guys!'))
loop.create_task(client.download_profile_photo('TelethonChat'))
They will run in the background as long as the loop is running too.
You can also `start an asyncio server
<https://docs.python.org/3/library/asyncio-stream.html#asyncio.start_server>`_
in the main script, and from another script, `connect to it
<https://docs.python.org/3/library/asyncio-stream.html#asyncio.open_connection>`_
to achieve `Inter-Process Communication
<https://en.wikipedia.org/wiki/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
<telethon.client.auth.AuthMethods.start>`, `run_until_disconnected
<telethon.client.updates.UpdateMethods.run_until_disconnected>`, and
`disconnect <telethon.client.telegrambaseclient.TelegramBaseClient.disconnect>`
all support this.
Where can I read more?
======================
`Check out my blog post
<https://lonamiwebs.github.io/blog/asyncio/>`_ about asyncio_, which
has some more examples and pictures to help you understand what happens
when the loop runs.
.. _asyncio: https://docs.python.org/3/library/asyncio.html