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 this per request
        self.cache = cache

    def __call__(self, url, num_retries...