.. _intro-overview:

==================
Scrapy at a glance
==================

Scrapy (/ˈskreɪpaɪ/) 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 `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.

Here's the code for a spider that scrapes famous quotes from website
https://quotes.toscrape.com, following the pagination:

.. code-block:: python

    import scrapy


    class QuotesSpider(scrapy.Spider):
        name = "quotes"
        start_urls = [
            "https://quotes.toscrape.com/tag/humor/",
        ]

        def parse(self, response):
            for quote in response.css("div.quote"):
                yield {
                    "author": quote.xpath("span/small/text()").get(),
                    "text": quote.css("span.text::text").get(),
                }

            next_page = response.css('li.next a::attr("href")').get()
            if next_page is not None:
                yield response.follow(next_page, self.parse)

Put this in a text file, name it something like ``quotes_spider.py``
and run the spider using the :command:`runspider` command::

    scrapy runspider quotes_spider.py -o quotes.jsonl

When this finishes you will have in the ``quotes.jsonl`` file a list of the
quotes in JSON Lines format, containing the text and author, which will look like this::

    {"author": "Jane Austen", "text": "\u201cThe person, be it gentleman or lady, who has not pleasure in a good novel, must be intolerably stupid.\u201d"}
    {"author": "Steve Martin", "text": "\u201cA day without sunshine is like, you know, night.\u201d"}
    {"author": "Garrison Keillor", "text": "\u201cAnyone who thinks sitting in church can make you a Christian must also think that sitting in a garage can make you a car.\u201d"}
    ...


What just happened?
-------------------

When you ran the command ``scrapy runspider quotes_spider.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 quotes in the *humor* category)
and called the default callback method ``parse``, passing the response object as
an argument. In the ``parse`` callback, we loop through the quote elements
using a CSS Selector, yield a Python dict with the extracted quote text and author,
look for a link to the next page and schedule another request using the same
``parse`` method as callback.

Here you will notice one of the main advantages of 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 a request fails or an
error happens while handling it.

While this enables you to do very fast crawls (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, limiting the amount of concurrent requests per domain or per IP, and
even :ref:`using an auto-throttling extension <topics-autothrottle>` that tries
to figure these settings out automatically.

.. 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.


.. _topics-whatelse:

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 extended CSS selectors and XPath expressions,
  with helper methods for extraction using regular expressions.

* An :ref:`interactive shell console <topics-shell>` (IPython aware) for trying
  out the CSS and XPath expressions to scrape data, which is very useful when writing or
  debugging your spiders.

* 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)

* Robust encoding support and auto-detection, for dealing with foreign,
  non-standard and broken encoding declarations.

* :ref:`Strong extensibility support <extending-scrapy>`, allowing you to plug
  in your own functionality using :ref:`signals <topics-signals>` and a
  well-defined API (middlewares, :ref:`extensions <topics-extensions>`, and
  :ref:`pipelines <topics-item-pipeline>`).

* A wide range of built-in extensions and middlewares for handling:

  - cookies and session handling
  - HTTP features like compression, authentication, caching
  - user-agent spoofing
  - robots.txt
  - crawl depth restriction
  - and more

* A :ref:`Telnet console <topics-telnetconsole>` for hooking into a Python
  console running inside your Scrapy process, to introspect and debug your
  crawler

* Plus other goodies like reusable spiders to crawl sites from `Sitemaps`_ and
  XML/CSV feeds, a media pipeline for :ref:`automatically downloading images
  <topics-media-pipeline>` (or any other media) associated with the scraped
  items, a caching DNS resolver, and much more!

What's next?
============

The next steps for you are to :ref:`install Scrapy <intro-install>`,
:ref:`follow through the tutorial <intro-tutorial>` to learn how to create
a full-blown Scrapy project and `join the community`_. Thanks for your
interest!

.. _join the community: https://scrapy.org/community/
.. _web scraping: https://en.wikipedia.org/wiki/Web_scraping
.. _Amazon Associates Web Services: https://affiliate-program.amazon.com/gp/advertising/api/detail/main.html
.. _Amazon S3: https://aws.amazon.com/s3/
.. _Sitemaps: https://www.sitemaps.org/index.html