Scrapy – A Beginner’s Guide
Scrapy is one of the most powerful, widely used web scraping libraries. It provides a “batteries included” approach to scraping, meaning it handles a lot of the common functionality that all scrapers need so developers can focus on building their applications.
Its spiders can be very generic or highly customizable, depending on the project’s needs. It also supports items pipelines to help scrapers avoid duplicate data, save it in CSV or SQLite, and much more.
The Scrapy API can be accessed through its command line interface, making it easy to get started with scraping and creating new spiders. Its built-in logging is useful for monitoring the crawler, and it’s also possible to collect statistics from the crawler, send email notifications about specific events, and more.
In addition, it provides tools for testing the behavior of web pages, and a web-crawling shell that lets you test the page’s components to see how they would behave in different scenarios.
A typical scrapy project consists of two main parts, the spider and the settings module. The spider stores the information about the website it is scraping, and the settings module contains the code that controls what the spider will do when it’s working.
When a web page is visited, a request is sent to the engine, which then dispatches it to the downloader. The downloader then downloads the requested webpage, generates a response, and sends it back to the engine.
Once the response is received, the engine sends a callback to the spider, which then performs the required actions on the response and yields more requests or data points. This callback is often called by other spiders that are scraping the same website.
In this way, each spider can send n requests to the engine at any given time (the limit of your hardware’s processing capability), and the scheduler keeps adding these requests to its queue so that it can dispatch them when asked.
You can configure the scrapy_spiders configuration variable to set allowed domains, which helps you restrict the spider from crawling unnecessary sites. You can also set a fixed delay between scraped pages, which will help the spider avoid overloading the server and causing problems for the site’s administrators.
If you’re working with a large crawler, it’s important to tune the Scrapy configuration so that it can run at the fastest speed it can while still running efficiently. This can be done by using a scheduler and an AutoThrottle feature, both of which are available in the configuration file.
It’s important to use XPath expressions and CSS selectors when writing a web crawler in Python. XPath is a powerful language for selecting elements on a web page, and Scrapy makes it easy to use it with its built-in pyXpath.
This makes it a breeze to write code that can be reused across multiple scraping projects. The beauty of XPath expressions is that they can be chained and used together to select a wide range of HTML elements on a page.