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 that parameters can be set once in the constructor and reused multiple times. Here is the updated implementation to support this:

class Downloader:
    def __init__(self, delay=5, user_agent='wswp', proxies=None, num_retries=1, cache=None):
        self.throttle = Throttle(delay)
        self.user_agent = user_agent
        self.proxies = proxies
        self.num_retries = num_retries
        self.cache = cache

    def __call__(self, url):
        result = None
        if self.cache:
            try:
               ...