asyncio

Scrapy has partial support for asyncio. After you install the asyncio reactor, you may use asyncio and asyncio-powered libraries in any coroutine.

Installing the asyncio reactor

To enable asyncio support, your TWISTED_REACTOR setting needs to be set to 'twisted.internet.asyncioreactor.AsyncioSelectorReactor', which is the default value.

If you are using AsyncCrawlerRunner or CrawlerRunner, you also need to install the AsyncioSelectorReactor reactor manually. You can do that using install_reactor():

install_reactor("twisted.internet.asyncioreactor.AsyncioSelectorReactor")

Handling a pre-installed reactor

twisted.internet.reactor and some other Twisted imports install the default Twisted reactor as a side effect. Once a Twisted reactor is installed, it is not possible to switch to a different reactor at run time.

If you configure the asyncio Twisted reactor and, at run time, Scrapy complains that a different reactor is already installed, chances are you have some such imports in your code.

You can usually fix the issue by moving those offending module-level Twisted imports to the method or function definitions where they are used. For example, if you have something like:

from twisted.internet import reactor


def my_function():
    reactor.callLater(...)

Switch to something like:

def my_function():
    from twisted.internet import reactor

    reactor.callLater(...)

Alternatively, you can try to manually install the asyncio reactor, with install_reactor(), before those imports happen.

Integrating Deferred code and asyncio code

Coroutine functions can await on Deferreds by wrapping them into asyncio.Future objects. Scrapy provides two helpers for this:

scrapy.utils.defer.deferred_to_future(d: Deferred[_T]) → Future[_T][source]

Return an asyncio.Future object that wraps d.

This function requires AsyncioSelectorReactor to be installed.

When using the asyncio reactor, you cannot await on Deferred objects from Scrapy callables defined as coroutines, you can only await on Future objects. Wrapping Deferred objects into Future objects allows you to wait on them:

class MySpider(Spider):
    ...
    async def parse(self, response):
        additional_request = scrapy.Request('https://example.org/price')
        deferred = self.crawler.engine.download(additional_request)
        additional_response = await deferred_to_future(deferred)

Changed in version 2.14: This function no longer installs an asyncio loop if called before the Twisted asyncio reactor is installed. A RuntimeError is raised in this case.

scrapy.utils.defer.maybe_deferred_to_future(d: Deferred[_T]) → Deferred[_T] | Future[_T][source]

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 TWISTED_REACTOR:

If you want to write code that uses Deferred objects but works with any reactor, use this function on all Deferred objects:

class MySpider(Spider):
    ...
    async def parse(self, response):
        additional_request = scrapy.Request('https://example.org/price')
        deferred = self.crawler.engine.download(additional_request)
        additional_response = await maybe_deferred_to_future(deferred)

Tip

If you don’t need to support reactors other than the default AsyncioSelectorReactor, you can use deferred_to_future(), otherwise you should use maybe_deferred_to_future().

Tip

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.

Coroutines and futures can be wrapped into Deferreds (for example, when a Scrapy API requires passing a Deferred to it) using the following helpers:

scrapy.utils.defer.deferred_from_coro(o: Awaitable[_T]) → Deferred[_T][source]
scrapy.utils.defer.deferred_from_coro(o: _T2) → _T2

Convert a coroutine or other awaitable object into a Deferred, or return the object as is if it isn’t a coroutine.

scrapy.utils.defer.deferred_f_from_coro_f(coro_f: Callable[_P, Awaitable[_T]]) → Callable[_P, Deferred[_T]][source]

Convert a coroutine function into a function that returns a Deferred.

The coroutine function will be called at the time when the wrapper is called. Wrapper args will be passed to it. This is useful for callback chains, as callback functions are called with the previous callback result.

scrapy.utils.defer.ensure_awaitable(o: Awaitable[_T], _warn: str | None = None) → Awaitable[_T][source]
scrapy.utils.defer.ensure_awaitable(o: _T, _warn: str | None = None) → Awaitable[_T]

Convert any value to an awaitable object.

For a Deferred object, use maybe_deferred_to_future() to wrap it into a suitable object. For an awaitable object of a different type, return it as is. For any other value, return a coroutine that completes with that value.

Added in version 2.14.

Enforcing asyncio as a requirement

If you are writing a component that requires asyncio to work, use scrapy.utils.asyncio.is_asyncio_available() to enforce it as a requirement. For example:

from scrapy.utils.asyncio import is_asyncio_available


class MyComponent:
    def __init__(self):
        if not is_asyncio_available():
            raise ValueError(
                f"{MyComponent.__qualname__} requires the asyncio support. "
                f"Make sure you have configured the asyncio reactor in the "
                f"TWISTED_REACTOR setting. See the asyncio documentation "
                f"of Scrapy for more information."
            )
scrapy.utils.asyncio.is_asyncio_available() → bool[source]

Check if it’s possible to call asyncio code that relies on the asyncio event loop.

Added in version 2.14.

This function returns True if there is a running asyncio event loop. If there is no such loop, it returns True if the Twisted reactor that is installed is AsyncioSelectorReactor, returns False if a different reactor is installed, and raises a RuntimeError if no reactor is installed.

Code that doesn’t directly require a Twisted reactor should use this function while code that requires AsyncioSelectorReactor should use is_asyncio_reactor_installed().

When this returns True, an asyncio loop is installed and used by Scrapy. It’s possible to call functions that require it, such as asyncio.sleep(), and await on asyncio.Future objects in Scrapy-related code.

When this returns False, a non-asyncio Twisted reactor is installed. It’s not possible to use asyncio features that require an asyncio event loop or await on asyncio.Future objects in Scrapy-related code, but it’s possible to await on Deferred objects.

Note

As this function uses asyncio.get_running_loop(), it will only detect the event loop if called in the same thread and from the code that runs inside that loop (this shouldn’t be a problem when calling it from code such as spiders and Scrapy components, if Scrapy is run using one of the supported ways).

Changed in version 2.15.0: This function now also returns True if there is a running asyncio loop, even if no Twisted reactor is installed.

scrapy.utils.reactor.is_asyncio_reactor_installed() → bool[source]

Check whether the installed reactor is AsyncioSelectorReactor.

Raise a RuntimeError if no reactor is installed.

In a future Scrapy version, when Scrapy supports running without a Twisted reactor, this function won’t be useful for checking if it’s possible to use asyncio features, so the code that that doesn’t directly require a Twisted reactor should use scrapy.utils.asyncio.is_asyncio_available() instead of this function.

Changed in version 2.13: In earlier Scrapy versions this function silently installed the default reactor if there was no reactor installed. Now it raises an exception to prevent silent problems in this case.

Using Scrapy without a Twisted reactor

Added in version 2.15.0.

Warning

This is currently experimental and may not be suitable for production use.

It’s possible to use Scrapy without installing a Twisted reactor at all, by setting the TWISTED_REACTOR_ENABLED setting to False. In this mode Scrapy will use the asyncio event loop directly, and most of the Scrapy functionality will work in the same way.

Doing this provides several benefits in certain use cases:

  • A Twisted reactor, once stopped, cannot be started again. This prevents, for example, using several instances of AsyncCrawlerProcess in the same process when they use a reactor, but with TWISTED_REACTOR_ENABLED=False it becomes possible.

  • There may be limitations imposed by AsyncioSelectorReactor and related Twisted code, such as the requirement of using SelectorEventLoop on Windows (see Windows-specific notes), that do not apply if the reactor is not used.

  • AsyncioSelectorReactor manages the underlying event loop, and while AsyncCrawlerRunner can use a pre-existing reactor which, in turn, can use a pre-existing event loop, it’s easier to use AsyncCrawlerRunner with a pre-existing loop directly.

  • Omitting the reactor machinery may improve performance and reliability.

Limitations

As some Scrapy features and components require a reactor, they don’t work and are disabled without it. Replacements that don’t require a reactor may be added in future Scrapy versions. The following features are not available:

Note that importing Twisted modules and, among other things, creating and using Deferred objects doesn’t require a reactor, so code that uses Deferred, Failure and some other Twisted APIs will not necessarily stop working.

Other differences

When TWISTED_REACTOR_ENABLED is set to False, Scrapy will change the defaults of some other settings:

Thus, HttpxDownloadHandler is used by default for making HTTP(S) requests. Please refer to its documentation for its differences and limitations compared to HTTP11DownloadHandler.

Additionally, AsyncCrawlerProcess will install a meta path finder that prevents twisted.internet.reactor from being imported.

Adding support to existing code

Code that doesn’t directly use Twisted APIs or APIs that depend on Twisted ones doesn’t need special support for running without a reactor.

Here are some examples of APIs and patterns that need a replacement:

Scrapy provides unified helpers for some of these examples:

scrapy.utils.asyncio.call_later(delay: float, func: Callable[[Unpack[_Ts]], object], *args: Unpack[_Ts]) → CallLaterResult[source]

Schedule a function to be called after a delay.

This uses either asyncio.loop.call_later() or reactor.callLater(), depending on whether asyncio support is available.

Added in version 2.14.0.

scrapy.utils.asyncio.create_looping_call(func: ~collections.abc.Callable[[~_P], ~scrapy.utils.asyncio._T], *args: ~typing.~_P, **kwargs: ~typing.~_P) → AsyncioLoopingCall | LoopingCall[source]

Create an instance of a looping call class.

This creates an instance of AsyncioLoopingCall or LoopingCall, depending on whether asyncio support is available.

Added in version 2.14.0.

class scrapy.utils.asyncio.AsyncioLoopingCall(func: ~collections.abc.Callable[[~_P], ~scrapy.utils.asyncio._T], *args: ~typing.~_P, **kwargs: ~typing.~_P)[source]

A simple implementation of a periodic call using asyncio, keeping some API and behavior compatibility with LoopingCall.

The function is called every interval seconds, independent of the finish time of the previous call. If the function is still running when it’s time to call it again, calls are skipped until the function finishes.

The function must not return a coroutine or a Deferred.

Added in version 2.14.0.

async scrapy.utils.asyncio.run_in_thread(func: ~collections.abc.Callable[[~_P], ~scrapy.utils.asyncio._T], *args: ~typing.~_P, **kwargs: ~typing.~_P) → _T[source]

Call a function in a thread and return its result as a coroutine.

This uses either asyncio.to_thread() or twisted.internet.threads.deferToThread(), depending on whether asyncio support is available.

Added in version 2.15.0.

If your code needs to know whether the reactor is available, you can either check for the value of the TWISTED_REACTOR_ENABLED setting (you need access to the Crawler instance to do this) or use the following function:

scrapy.utils.reactorless.is_reactorless() → bool[source]

Check if we are running in the reactorless mode, i.e. with TWISTED_REACTOR_ENABLED set to False.

As this checks the runtime state and not the setting itself, it can be wrong when executed very early, before the reactor and/or the asyncio event loop are initialized.

Note

As this function uses scrapy.utils.asyncio.is_asyncio_available(), it has the same limitations for detecting a running asyncio event loop as that one.

Added in version 2.15.0.

In general, code that doesn’t use the reactor (directly or indirectly) can be used unmodified both with the asyncio reactor and without a reactor. This includes code that converts Deferreds to futures and vice versa as described in Integrating Deferred code and asyncio code.

Troubleshooting

ImportError: Import of twisted.internet.reactor is forbidden when running without a Twisted reactor […]: Scrapy is configured to run without a reactor, but some code imported twisted.internet.reactor, most likely because that code needs a reactor to be used. You need to stop using this code or set TWISTED_REACTOR_ENABLED back to True. It’s also possible that the reactor isn’t really needed but was installed due to the problem described in Handling a pre-installed reactor, in which case it should be enough to fix the problematic imports.

RuntimeError: TWISTED_REACTOR_ENABLED is False but a Twisted reactor is installed: Scrapy is configured to run without a reactor, but a reactor is already installed before the Scrapy code is executed. If you are trying to set TWISTED_REACTOR_ENABLED via per-spider settings, it’s currently unsupported.

RuntimeError: We expected a Twisted reactor to be installed but it isn’t: Scrapy is configured to run with a reactor and not to install one, but a reactor wasn’t installed before the Scrapy code is executed. If you are trying to set TWISTED_REACTOR_ENABLED via per-spider settings, it’s currently unsupported.

RuntimeError: <class> doesn’t support TWISTED_REACTOR_ENABLED=False: The listed class cannot be used with TWISTED_REACTOR_ENABLED set to False. There may be a replacement in the documentation above or the documentation of the affected class.

Windows-specific notes

The Windows implementation of asyncio can use two event loop implementations, ProactorEventLoop (default) and SelectorEventLoop. However, only SelectorEventLoop works with Twisted.

Scrapy changes the event loop class to SelectorEventLoop automatically when you change the TWISTED_REACTOR setting or call install_reactor().

Note

Other libraries you use may require ProactorEventLoop, e.g. because it supports subprocesses (this is the case with playwright), so you cannot use them together with Scrapy on Windows (but you should be able to use them on WSL or native Linux).

Note

This problem doesn’t apply when not using the reactor, see Using Scrapy without a Twisted 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.

Switching to a non-asyncio reactor

If for some reason your code doesn’t work with the asyncio reactor, you can use a different reactor by setting the TWISTED_REACTOR setting to its import path (e.g. 'twisted.internet.epollreactor.EPollReactor') or to None, which will use the default reactor for your platform. If you are using AsyncCrawlerRunner or AsyncCrawlerProcess you also need to switch to their Deferred-based counterparts: CrawlerRunner or CrawlerProcess respectively.