5.3 KiB
asyncio
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.. versionadded:: 2.0
Scrapy has partial support for :mod:`asyncio`. After you :ref:`install the asyncio reactor <install-asyncio>`, you may use :mod:`asyncio` and :mod:`asyncio`-powered libraries in any :doc:`coroutine <coroutines>`.
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Unknown interpreted text role "doc".Installing the asyncio reactor
To enable :mod:`asyncio` support, your :setting:`TWISTED_REACTOR` setting needs to be set to 'twisted.internet.asyncioreactor.AsyncioSelectorReactor', which is the default value.
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Unknown interpreted text role "setting".If you are using :class:`~scrapy.crawler.CrawlerRunner`, you also need to install the :class:`~twisted.internet.asyncioreactor.AsyncioSelectorReactor` reactor manually. You can do that using :func:`~scrapy.utils.reactor.install_reactor`:
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.. code-block:: python
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 :ref:`configure the asyncio Twisted reactor <install-asyncio>` and, at run time, Scrapy complains that a different reactor is already installed, chances are you have some such imports in your code.
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Unknown interpreted text role "ref".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:
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.. code-block:: python
from twisted.internet import reactor
def my_function():
reactor.callLater(...)
Switch to something like:
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.. code-block:: python
def my_function():
from twisted.internet import reactor
reactor.callLater(...)
Alternatively, you can try to :ref:`manually install the asyncio reactor <install-asyncio>`, with :func:`~scrapy.utils.reactor.install_reactor`, before those imports happen.
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Unknown interpreted text role "func".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 :class:`asyncio.Future` objects, not into :class:`~twisted.internet.defer.Deferred` directly), and you need to wrap them into Futures. Scrapy provides two helpers for this:
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.. autofunction:: scrapy.utils.defer.deferred_to_future
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.. autofunction:: scrapy.utils.defer.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 :mod:`asyncio`), you can copy the implementation of these functions into your own code.
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Unknown interpreted text role "mod".Enforcing asyncio as a requirement
If you are writing a :ref:`component <topics-components>` that requires asyncio to work, use :func:`scrapy.utils.reactor.is_asyncio_reactor_installed` to :ref:`enforce it as a requirement <enforce-component-requirements>`. For example:
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.. code-block:: python
from scrapy.utils.reactor import is_asyncio_reactor_installed
class MyComponent:
def __init__(self):
if not is_asyncio_reactor_installed():
raise ValueError(
f"{MyComponent.__qualname__} requires the asyncio Twisted "
f"reactor. Make sure you have it configured in the "
f"TWISTED_REACTOR setting. See the asyncio documentation "
f"of Scrapy for more information."
)
Windows-specific notes
The Windows implementation of :mod:`asyncio` can use two event loop implementations, :class:`~asyncio.ProactorEventLoop` (default) and :class:`~asyncio.SelectorEventLoop`. However, only :class:`~asyncio.SelectorEventLoop` works with Twisted.
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Unknown interpreted text role "class".Scrapy changes the event loop class to :class:`~asyncio.SelectorEventLoop` automatically when you change the :setting:`TWISTED_REACTOR` setting or call :func:`~scrapy.utils.reactor.install_reactor`.
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Unknown interpreted text role "func".Note
Other libraries you use may require :class:`~asyncio.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).
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Unknown interpreted text role "class".Using custom asyncio loops
You can also use custom asyncio event loops with the asyncio reactor. Set the :setting:`ASYNCIO_EVENT_LOOP` setting to the import path of the desired event loop class to use it instead of the default asyncio event loop.
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Unknown interpreted text role "setting".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 :setting:`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.
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