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Clarify the docs about reactor settings (#7880)

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Adrian 2026-08-09 20:01:31 +02:00 committed by GitHub
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docs/topics/settings.rst
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@ -69,9 +69,10 @@ Example::
precedence and override the project ones.
.. note:: :ref:`Pre-crawler settings <pre-crawler-settings>` cannot be defined
per spider, and :ref:`reactor settings <reactor-settings>` should not have
a different value per spider when :ref:`running multiple spiders in the
same process <run-multiple-spiders>`.
per spider, and :ref:`reactor settings <reactor-settings>` and
:ref:`logging settings <logging-settings>` are subject to restrictions when
:ref:`running multiple spiders in the same process
<run-multiple-spiders>`.
One way to do so is by setting their :attr:`~scrapy.Spider.custom_settings`
attribute:
@ -329,32 +330,41 @@ Reactor settings
**Reactor settings** are settings tied to the :doc:`Twisted reactor
<twisted:core/howto/reactor-basics>`.
These settings can be defined from a spider. However, because only 1 reactor
can be used per process, these settings cannot use a different value per spider
when :ref:`running multiple spiders in the same process
<run-multiple-spiders>`.
Because only 1 reactor can be used per process, these settings cannot use a
different value per spider when :ref:`running multiple spiders in the same
process <run-multiple-spiders>`.
In general, if different spiders define different values, the first defined
value is used. However, if two spiders request a different reactor, an
exception is raised.
These settings are:
These settings are used upon installing the reactor:
- :setting:`ASYNCIO_EVENT_LOOP` (not possible to set per-spider when using
:class:`~scrapy.crawler.AsyncCrawlerProcess`, see below)
- :setting:`TWISTED_REACTOR` (ignored when using
:class:`~scrapy.crawler.AsyncCrawlerProcess`, see below)
They can be :ref:`set from a spider <spider-settings>`, but only the values
from the first spider that runs are used, since that is when the reactor is
installed. If a later spider asks for a different reactor or a different event
loop, an exception is raised. With
:class:`~scrapy.crawler.CrawlerRunner` and
:class:`~scrapy.crawler.AsyncCrawlerRunner` the reactor must be installed
beforehand, so these settings are only used to check that the installed reactor
and event loop match them.
These settings are applied when starting the reactor:
- :setting:`TWISTED_DNS_RESOLVER` and settings used by the corresponding
component, e.g. :setting:`DNSCACHE_ENABLED`, :setting:`DNSCACHE_SIZE`
and :setting:`DNS_TIMEOUT` for the default one.
- :setting:`REACTOR_THREADPOOL_MAXSIZE`
- :setting:`TWISTED_REACTOR` (ignored when using
:class:`~scrapy.crawler.AsyncCrawlerProcess`, see below)
:setting:`ASYNCIO_EVENT_LOOP` and :setting:`TWISTED_REACTOR` are used upon
installing the reactor. The rest of the settings are applied when starting
the reactor.
They are read from the settings of the
:class:`~scrapy.crawler.CrawlerProcess` or
:class:`~scrapy.crawler.AsyncCrawlerProcess` object, so setting them from a
spider or an :ref:`add-on <topics-addons>` has no effect. They are ignored
altogether when using :class:`~scrapy.crawler.CrawlerRunner` or
:class:`~scrapy.crawler.AsyncCrawlerRunner`, which do not start the reactor.
There is an additional restriction for :setting:`TWISTED_REACTOR` and
:setting:`ASYNCIO_EVENT_LOOP` when using