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Scrapy at a glance
Scrapy is an application framework for crawling web sites and extracting structured data which can be used for a wide range of useful applications, like data mining, information processing or historical archival.
Even though Scrapy was originally designed for screen scraping (more precisely, web scraping), it can also be used to extract data using APIs (such as Amazon Associates Web Services) or as a general purpose web crawler.
Walk-through of an example spider
In order to show you what Scrapy brings to the table, we'll walk you through an example of a Scrapy Spider using the simplest way to run a spider.
Once you're ready to dive in more, you can :ref:`follow the tutorial and build a full-blown Scrapy project <intro-tutorial>`.
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Unknown interpreted text role "ref".So, here's the code for a spider that follows the links to the top voted questions on StackOverflow and scrapes some data from each page:
import scrapy
class StackOverflowSpider(scrapy.Spider):
name = 'stackoverflow'
start_urls = ['http://stackoverflow.com/questions?sort=votes']
def parse(self, response):
for href in response.css('.question-summary h3 a::attr(href)'):
full_url = response.urljoin(href.extract())
yield scrapy.Request(full_url, callback=self.parse_question)
def parse_question(self, response):
yield {
'title': response.css('h1 a::text').extract()[0],
'votes': response.css('.question .vote-count-post::text').extract()[0],
'body': response.css('.question .post-text').extract()[0],
'tags': response.css('.question .post-tag::text').extract(),
'link': response.url,
}
Put this in a file, name it to something like stackoverflow_spider.py and run the spider using the :command:`runspider` command:
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Unknown interpreted text role "command".scrapy runspider stackoverflow_spider.py -o top-stackoverflow-questions.json
When this finishes you will have in the top-stackoverflow-questions.json file a list of the most upvoted questions in StackOverflow in JSON format, containing the title, link, number of upvotes, a list of the tags and the question content in HTML.
What just happened?
When you ran the command scrapy runspider somefile.py, Scrapy looked for a Spider definition inside it and ran it through its crawler engine.
The crawl started by making requests to the URLs defined in the start_urls attribute (in this case, only the URL for StackOverflow top questions page), and called the default callback method parse passing the response object as an argument. In the parse callback, we extract the links to the question pages using a CSS Selector with a custom extension that allows to get the value for an attribute. Then, we yield a few more requests to be sent, registering the method parse_question as the callback to be called for each of them as they finish.
Here you notice one of the main advantages about Scrapy: requests are :ref:`scheduled and processed asynchronously <topics-architecture>`. This means that Scrapy doesn't need to wait for a request to be finished and processed, it can send another request or do other things in the meantime. This also means that other requests can keep going even if some request fails or an error happens while handling it.
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Unknown interpreted text role "ref".While this enables you to do very fast crawlings sending multiple concurrent requests at the same time in a fault-tolerant way, Scrapy also gives you control over the politeness of the crawl through :ref:`a few settings <topics-settings-ref>`. You can do things like setting a download delay between each request, limit amount of concurrent requests per domain or per IP, and even :ref:`use an auto-throttling extension <topics-autothrottle>` that tries to figure out these automatically.
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Unknown interpreted text role "ref".Finally, the parse_question callback scrapes the question data for each page yielding a dict, which Scrapy then collects and writes to a JSON file as requested in the command line.
Note
This is using :ref:`feed exports <topics-feed-exports>` to generate the JSON file, you can easily change the export format (XML or CSV, for example) or the storage backend (FTP or Amazon S3, for example). You can also write an :ref:`item pipeline <topics-item-pipeline>` to store the items in a database.
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Unknown interpreted text role "ref".The data in the file will look like this (note: formatted for easier reading):
[{
"body": "... LONG HTML HERE ...",
"link": "http://stackoverflow.com/questions/11227809/why-is-processing-a-sorted-array-faster-than-an-unsorted-array",
"tags": ["java", "c++", "performance", "optimization"],
"title": "Why is processing a sorted array faster than an unsorted array?",
"votes": "9924"
},
{
"body": "... LONG HTML HERE ...",
"link": "http://stackoverflow.com/questions/1260748/how-do-i-remove-a-git-submodule",
"tags": ["git", "git-submodules"],
"title": "How do I remove a Git submodule?",
"votes": "1764"
},
...]
What else?
You've seen how to extract and store items from a website using Scrapy, but this is just the surface. Scrapy provides a lot of powerful features for making scraping easy and efficient, such as:
Built-in support for :ref:`selecting and extracting <topics-selectors>` data from HTML/XML sources using CSS selectors extended and XPath expressions, with helper methods to extract using regular expressions.
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An :ref:`interactive shell console <topics-shell>` (IPython aware) for trying out the CSS and XPath expressions to scrape data, very useful when writing or debugging your spiders.
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Built-in support for :ref:`generating feed exports <topics-feed-exports>` in multiple formats (JSON, CSV, XML) and storing them in multiple backends (FTP, S3, local filesystem)
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Robust encoding support and auto-detection, for dealing with foreign, non-standard and broken encoding declarations.
Strong :ref:`extensibility support <extending-scrapy>` and lots of built-in extensions and middlewares to handle things like cookies, crawl throttling, HTTP caching, HTTP compression, user-agent spoofing, robots.txt, stats collection and many more.
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A :ref:`Telnet console <topics-telnetconsole>` for hooking into a Python console running inside your Scrapy process, to introspect and debug your crawler
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Plus other goodies like reusable spiders to crawl sites from Sitemaps and XML/CSV feeds, a media pipeline for :ref:`automatically downloading images <topics-images>` (or any other media) associated with the scraped items, a caching DNS resolver, and much more!
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What's next?
The next obvious steps for you are to download Scrapy, read :ref:`the tutorial <intro-tutorial>` and join the community. Thanks for your interest!
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