Book Image

Python Web Scraping - Second Edition

By : Katharine Jarmul
Book Image

Python Web Scraping - Second Edition

By: Katharine Jarmul

Overview of this book

The Internet contains the most useful set of data ever assembled, most of which is publicly accessible for free. However, this data is not easily usable. It is embedded within the structure and style of websites and needs to be carefully extracted. Web scraping is becoming increasingly useful as a means to gather and make sense of the wealth of information available online. This book is the ultimate guide to using the latest features of Python 3.x to scrape data from websites. In the early chapters, you'll see how to extract data from static web pages. You'll learn to use caching with databases and files to save time and manage the load on servers. After covering the basics, you'll get hands-on practice building a more sophisticated crawler using browsers, crawlers, and concurrent scrapers. You'll determine when and how to scrape data from a JavaScript-dependent website using PyQt and Selenium. You'll get a better understanding of how to submit forms on complex websites protected by CAPTCHA. You'll find out how to automate these actions with Python packages such as mechanize. You'll also learn how to create class-based scrapers with Scrapy libraries and implement your learning on real websites. By the end of the book, you will have explored testing websites with scrapers, remote scraping, best practices, working with images, and many other relevant topics.
Table of Contents (10 chapters)

Adding cache support to the link crawler

To support caching, the download function developed in Chapter 1, Introduction to Web Scraping, needs to be modified to check the cache before downloading a URL. We also need to move throttling inside this function and only throttle when a download is made, and not when loading from a cache. To avoid the need to pass various parameters for every download, we will take this opportunity to refactor the download function into a class so parameters can be set in the constructor and reused numerous times. Here is the updated implementation to support this:

from chp1.throttle import Throttle
from random import choice
import requests


class Downloader:
def __init__(self, delay=5, user_agent='wswp', proxies=None, cache={}):
self.throttle = Throttle(delay)
self.user_agent = user_agent
self.proxies = proxies
self.num_retries = None # we will set...