New in version 2.0.
Installing the asyncio reactor¶
Using custom asyncio loops¶
You can also use custom asyncio event loops with the asyncio reactor. Set the
ASYNCIO_EVENT_LOOP setting to the import path of the desired event loop class to
use it instead of the default asyncio event loop.
The Windows implementation of
asyncio can use two event loop
SelectorEventLoop, default before Python 3.8, required when using Twisted.
ProactorEventLoop, default since Python 3.8, cannot work with Twisted.
So on Python 3.8+ the event loop class needs to be changed.
To change the event loop class manually, call the following code before installing the reactor:
import asyncio asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
You can put this in the same function that installs the reactor, if you do that
yourself, or in some code that runs before the reactor is installed, e.g.
Awaiting on Deferreds¶
When the asyncio reactor isn’t installed, you can await on Deferreds in the
coroutines directly. When it is installed, this is not possible anymore, due to
specifics of the Scrapy coroutine integration (the coroutines are wrapped into
asyncio.Future objects, not into
Deferred directly), and you need to wrap them into
Futures. Scrapy provides two helpers for this:
- scrapy.utils.defer.deferred_to_future(d: Deferred) Future ¶
New in version 2.6.0.
asyncio.Futureobject that wraps d.
When using the asyncio reactor, you cannot await on
Deferredobjects from Scrapy callables defined as coroutines, you can only await on
Futureobjects allows you to wait on them:
class MySpider(Spider): ... async def parse(self, response): d = treq.get('https://example.com/additional') additional_response = await deferred_to_future(d)
- scrapy.utils.defer.maybe_deferred_to_future(d: Deferred) Union[Deferred, Future] ¶
New in version 2.6.0.
Return d as an object that can be awaited from a Scrapy callable defined as a coroutine.
What you can await in Scrapy callables defined as coroutines depends on the value of
When not using the asyncio reactor, you can only await on
If you want to write code that uses
Deferredobjects but works with any reactor, use this function on all
class MySpider(Spider): ... async def parse(self, response): d = treq.get('https://example.com/additional') extra_response = await maybe_deferred_to_future(d)
If you need to use these functions in code that aims to be compatible
with lower versions of Scrapy that do not provide these functions,
down to Scrapy 2.0 (earlier versions do not support
asyncio), you can copy the implementation of these functions
into your own code.