scrapy-dynamic-spiders 1.0.0a1

Scrapy Dynamic Spiders

Scrapy configures its Crawler objects using class variables associated with each Spider class. Most of these can be meaningfully changed using the Spider's constructor, or even with other Spider class methods. However, some class variables, most notably custom_settings, are accessed before Spider instantiation. Crawl rules are similarly accessed before spider instantiation, though a protected method does allow for rule re-compilation with somewhat unpredictable results. In certain cases, this can lead to the creation of large numbers of subclasses made solely to handle slightly different websites or use cases.

Dynamic Spiders offers a simple, extensible API for generating spider subclasses based on existing ones, as well as an API for running crawls in a synchronous environment (using Crochet).

Use Cases

• Mutate your pipeline without putting a million boolean flags in your code
• Rapidly generate large numbers of spider subclasses based on external data
• Run crawls from within a blocking event loop
• Run crawls tailored to user input

The APIs

What follows is a brief overview of the provided APIs.

SpiderClsFactory

class scrapy_dynamic_spiders.factories.SpiderClsFactory(custom_settings=None, settings_ow=False)

This is the basic SpiderClsFactory, which can handle Spider, XmlFeedSpider, CsvFeedSpider, and SitemapSpider subclasses. It allows for dynamic definition of custom_settings, which is the only thing in these generic spider classes which have effects before instantiation.

Attributes

custom_settings - a custom_settings dictionary.
settings_ow - determines how custom_settings are handled. If True, the provided settings overwrite any other custom settings native to the template class. If False, the provided settings are merged with a copy of the native settings, with preference for provided settings.

Methods

construct_spiderself, (spidercls) -> type - construct_spider returns a subclass of the template spidercls with its custom settings class variable customized based on the Factory's current attributes.

Extension

You should subclass SpiderFactory if you wish to customize class variables besides custom_settings. This is done in the construct_spider method, which you will thus need to overwrite. There is one relevant private method which you may wish to use.

_construct_custom_settings(self, spidercls) -> dict - returns a new custom_settings dictionary based off of existing custom_settings and the Factory's current attributes.

CrawlSpiderClsFactory

Inherits from SpiderClsFactory.

class scrapy_dynamic_spiders.factories.CrawlSpiderClsFactory(custom_settings=None, settings_ow=False,
                                         extractor_configs=None, rule_configs=None, rule_ow:=False)

This subclass handles rule construction for CrawlSpiders and CrawlSpider subclasses. It uses two lists of dictionaries, extractor_configs and rule_configs to create rules dynamically. Each extractor_config dictionary provides kwargs to a LinkExtractor, whose corresponding Rule is provided keyword arguments by the dictionary at the matching index position in rule_configs. If there are fewer extractor_configs than rule_configs, the last extractor_config is used for all extra rule_configs.

Attributes

extractor_configs - a list of dictionaries. Each dictionary should contain kwargs for a LinkExtractor object.
rule_configs - a list of dictionaries. Each dictionary should contain kwargs for a Rule object.
rule_ow - if True, any exiting rules are replaced by the dynamically generated rules. If false, new rules are appended to a copy of the existing rule list.

Extension

Extending this subclass should be similar to extending SpiderFactory. There is one private method which you may wish to use.

_construct_rule_list(self, spidercls) -> List[Rule] - returns a new list of Rule objects based on those in the template spidercls and the Factory's current attributes.

SpiderWrangler

class scrapy_dynamic_spiders.wranglers.SpiderWrangler(settings, spidercls = None, clsfactory = None, gen_spiders = True)

This class allows you to run scrapy crawls sequentially, within a synchronous context. It does this through a crochet- managed Twisted Reactor. Because of this, it executes crawls using a composited CrawlerRunner object, rather than a CrawlerProcess object. SpiderWranglers offer built-in support for generating spider subclasses using a SpiderClsFactory, though this behavior is optional.

Parameters

settings - a Scrapy Settings object. Required to initialize the composited CrawlerRunner.

Attributes

gen_spiders - If True, crawls made using the start_crawl method will use a spider subclass generated using the composited SpiderClsFactory. If False, crawls will use the Spider class referenced in the Wrangler's spidercls attribute. Attempting to crawl with gen_spiders=True without a clsfactory raises an error.

spidercls - The Spider class which will be used for crawling or generating subclasses for crawling.

clsfactory - A SpiderClsFactory or subclass instance.

Methods

start_crawl(self, *args, **kwargs) -> Deferred - Initiates a crawl which blocks in a synchronous context. Unlike Scrapy's Core API, this allows you to perform multiple crawls in sequence without having to manually manipulate your Reactor. *args and **kwargs are passed to the Spider's constructor. The Spider class used is either spidercls or a dynamically-generated subclass of it.

Extension

Extension of SpiderWrangler is generally straightforward. However, modifying the behavior of start_crawl requires extra attention. start_crawl is decorated using crochet's wait_for decorator. To modify the timeout specified by this decorator, you will need to override the existing start_crawl method. To simplify this, the primary functionality of start_crawl is encapusalated in the private method _start_crawl. Changing the behavior of the decorator thus requires overriding start_crawl with a new instance of the wait_for decorator, and calling _start_crawl in the body of the function.

_start_crawl(self, *args, **kwargs) -> Deferred - Wraps the actual functionality of start_crawl. Return the value return by this function, or a mutation of it, when overriding start_crawl.

Tests

Run tests like so.

python -m unittests tests

Known Issues

• Running tests results in unclosed sockets. Currently investigating.

Changelog

v1.0.0a1

• Initial alpha release