The Hacker's Guide to Scaling Python/

...

/

Using Asyncio

Using Asyncio

Learn about Asyncio module to handle blocking calls.

We'll cover the following...

Now that you have a good idea of what an event loop is and of what it provides, it is a good time to dig into the use of a state-of-the-art event loop. Asyncio is new in Python 3, and it requires Python 3.5 or later to use it.

Event loop in asyncio

Asyncio is centered on the concept of event loops, which work in the same way as the select module (described in the previous lesson). Once asyncio has created an event loop, an application registers the functions to call back when a specific event happens: as time passes, a file descriptor is ready to be read, or a socket is ready to be written.

Coroutine lifecycle

That type of function is called a coroutine. It is a particular type of function that can give back control to the caller so that the event loop can continue running. It works in the same manner that a generator would, giving back the control to a caller using the yield statement.

Press + to interact
import asyncio
async def hello_world():
    print("hello world!")
    return 42
hello_world_coroutine = hello_world()
print(hello_world_coroutine)
event_loop = asyncio.get_event_loop()
try:
    print("entering event loop")
    result = event_loop.run_until_complete(hello_world_coroutine)
    print(result)
finally:
    event_loop.close()

The above example shows a very straightforward implementation of an event loop using a coroutine. The coroutine hello_world is defined as a function, except that the keyword to start its definition is async def rather than just def. This coroutine just prints a message and returns a result.

The event loop runs this coroutine and is terminated as soon as the coroutine returns, ending the program. Coroutines ...