sep/sep-019.rst
======= =================== SEP 19 Title Per-spider settings Author Pablo Hoffman, Nicolás Ramirez, Julia Medina Created 2013-03-07 Status Final (implemented with minor variations) ======= ===================
This is a proposal to add support for overriding settings per-spiders in a consistent way, while taking the chance to refactor the settings population and whole crawl workflow.
In short, you will be able to overwrite settings (on a per-spider basis) by implementing a class method in your spider:
.. code-block:: python
class MySpider(Spider):
@classmethod
def custom_settings(cls):
return {
"DOWNLOAD_DELAY": 5.0,
"RETRY_ENABLED": False,
}
custom_settings class method will be added to spiders, to give them
a chance to override settings before they're used to instantiate the crawlerload method that will return a spider class given its name), but
spider initialization will be delegated to crawlers (with a new
from_crawler class method in spiders, that will allow them access to
crawlers directly)SPIDER_MODULES and SPIDER_MANAGER_CLASS settings will be removed and
replaced by entries on scrapy.cfg. Thus spider managers won't need
project settings to configure themselvesSettingsReader instanceSettingsLoader and
SettingsReader, and a new concept of "setting priority" will be addedSettings class will be split into two classes SettingsLoader and
SettingsReader. First one will be used to settle all different levels of
settings across the project, and the later will be a frozen version of the
already loaded settings and will be the preferred way to access them. This
will avoid the current possible misconception that you can change settings
after they have been populated. There'll be a new concept of settings
priorities, and settings.overrides will be deprecated in favor of
explicitly loaded settings with priorities, as it'll make the settings
overriding not order-dependent.
Because of this, CrawlerSettings (with its overrides, settings_module and
defaults) will be remove, but its interface could be maintained for backward
compatibility in SettingsReader (since on SettingsLoader, overrides
dictionary and settings with priorities don't get along with a consistent
implementation). Maintaining this attributes and their functionality is not
advisable since it breaks the read-only access nature of the class.
With the new per-spider settings, there's a need of a helper function that
will take a spider and return a SettingsReader instance populated with
defaults', project's and the given spider's settings. Motive behind this is
that get_project_settings can't continue being used for getting settings
instance for crawler usage when using the API directly, as the project is not
the only source of settings anymore. get_projects_settings will become an
internal function because of that.
SettingsLoader is going to populate settings at startup, then it'll be
converted to a SettingsReader instance and discarded afterwards.
It is supposed to be write-only, but many previously loaded settings are
needed to be accessed before freezing them. For example, the
COMMANDS_MODULE setting allows loading more command default settings.
Another example is that we need to read LOG_* settings early because we
must be able to log errors on the load settings process. ScrapyCommands
may be configured based upon current settings, as users can plug custom
commands. These are some of the reasons that suggest that we need a read-write
access for this class.
Will have a method set(name, value, priority) to register a setting with
a given priority. A setdict(dict, priority) method may come handy for
loading project's and per-spider settings.
Will have current Settings getter functions (get, getint,
getfloat, getdict, etc.) (See above for reasons behind this).
Will have a freeze method that returns an instance of
SettingsReader, with a copy of the current state of settings (already
prioritized).
It's intended to be the one used by core, extensions, and all components that
use settings without modifying them. Because there are logical objects that
change settings, such as ScrapyCommands, use cases of each settings class
need to be comprehensively explained.
New crawlers will be created with an instance of this class (The one returned
by the freeze method on the already populated SettingsReader), because
they are not expected to alter the settings.
It'll be read-only, keeping the same getter methods of current Settings
(get, getint, getfloat, getdict, etc.). There could be a
set method that will throw a descriptive explanatory error for debugging
compatibility, avoiding its inadvertently usage.
There will be 5 setting priorities used by default:
scrapy.settings.default_settings)KEEP_ALIVE=True)settings.py)Spider.custom_settings class method)There are a couple of issues here:
SCRAPY_PICKLED_SETTINGS_TO_OVERRIDE and SCRAPY_{settings} need-to-be
deprecated environment variables: Can be kept, with a new or existing
priority.
We could have different priorities for settings given with the -s
argument and other named arguments in the command line (For example, -s LOG_ENABLE=False --loglevel=ERROR will set LOG_ENABLE to True, because
named options are overridden later in the current implementation), but
because the processing of command line options is done in one place we could
leave them with the same priority and depend on the order of the set calls
just for this case.
scrapy.conf.settings singleton is a deprecated implementation concerning
settings load. Could be maintained as it is, but the singleton should
implement new SettingsReader interface in order to work.
Currently, the spider manager is part of the crawler which creates a cyclic loop between settings and spiders and it shouldn't belong there. The spiders should be loaded outside and passed to the crawler object, which will require a spider class to be instantiated.
This new spider manager will not have access to the settings (they won't be loaded yet) so it will use scrapy.cfg to configure itself.
The scrapy.cfg would look like this::
[settings]
default = myproject.settings
[spiders]
manager = scrapy.spidermanager.SpiderManager
modules = myproject.spiders
manager replaces SPIDER_MANAGER_CLASS setting and will, if omitted,
default to scrapy.spidermanager.SpiderManagermodules replaces SPIDER_MODULES setting and will be requiredThese ideas translate to the following changes on the SpiderManager class:
__init__(spider_modules) -> __init__(). spider_modules will be
looked in scrapy.cfg.
create('spider_name', **spider_kargs) -> load('spider_name'). This
will return a spider class, not an instance. It's basically a __get__
to self._spiders.
All remaining functions should be deprecated or remove accordingly, since a crawler reference is no longer needed.
New helper get_spider_manager_class_from_scrapycfg in
scrapy/utils/spidermanager.py.
A new class method custom_settings is proposed, that could be use to
override project and default settings before they're used to instantiate the
crawler:
.. code-block:: python
class MySpider(Spider):
@classmethod
def custom_settings(cls):
return {
"DOWNLOAD_DELAY": 5.0,
"RETRY_ENABLED": False,
}
This will only involve a set call with the corresponding priority when
populating SettingsLoader.
Contributing to API changes, new from_crawler class method will be added
to spiders to give them a chance to access settings, stats, or the crawler
core components themselves. This should be the new way to create a spider from
now on (instead of normally instantiate it as is currently).
As already stated, ScrapyCommands modify the settings, so they need a
SettingsLoader instance reference in order to do that.
Present process_option implementations on Base and other commands read and
override settings. These overrides should be changed to set calls with
the allocated priority.
Each command with a custom run method should be modified to reflect the new
refactored API (Particularly crawl command).
CrawlerProcess should be remove because Scrapy crawl command no longer
supports running multiple spiders. The preferred way for doing this is using
the API manually, instantiating a separate Crawler for each spider, so
CrawlerProcess has loosen its utility.
This change is not directly related to the project (it's not focus on settings but it fits in the API clean up task), but it's a great opportunity to consider since we're changing the crawling startup flow.
This class will be deleted and the attributes and methods will be merge with
Crawler. For that effect, these are the specific merges and removals:
self.crawlers doesn't make sense is this new set up, each reference will
be replace with self.
create_crawler will be __init__ of Crawler
_start_crawler will be merge with Crawler.start
start will be merge with Crawler.crawl but this will need from the
later an extra boolean parameter start_reactor (default: True) to crawl
with or without starting twisted reactor (This is required in
commands.shell in order to start the reactor in another thread).
This summarizes the current and new proposed mechanisms for starting up a Scrapy crawler. Imports and non representative functions are omitted for brevity.
::
# execute in cmdline
# loads settings.py, returns CrawlerSettings(settings_module)
settings = get_project_settings()
settings.defaults.update(cmd.default_settings)
cmd.crawler_process = CrawlerProcess(settings)
cmd.run # (In a _run_print_help call)
# Command.run in commands/crawl.py
self.crawler_process.create_crawler()
spider = crawler.spiders.create(spider_name, **spider_kwargs)
crawler.crawl(spider)
self.crawler_process.start() # starts crawling spider
# CrawlerProcess._start_crawler in crawler.py
crawler.configure()
::
# execute in cmdline
smcls = get_spider_manager_class_from_scrapycfg()
sm = smcls() # loads spiders from module defined in scrapy.cfg
spidercls = sm.load(spider_name) # returns spider class, not instance
settings = get_project_settings() # loads settings.py
settings.setdict(cmd.default_settings, priority=40)
settings.setdict(spidercls.custom_settings(), priority=30)
settings = settings.freeze()
cmd.crawler = Crawler(spidercls, settings=settings)
# Crawler.__init__ in crawler.py
self.configure()
cmd.run # (In a _run_print_help call)
# Command.run in commands/crawl.py
self.crawler.crawl(**spider_kwargs)
# Crawler.crawl in crawler.py
spider = self.spidercls.from_crawler(self, **spider_kwargs)
# starts crawling spider