How do I handle relative URLs when scraping with lxml?

When scraping web pages with lxml, you might encounter relative URLs in the source HTML. Relative URLs are URLs that are not complete paths and are relative to the current document's location. To handle relative URLs properly, you need to resolve them to absolute URLs before you can access the resources they point to. Here's how you can do that:

1. Parse the base URL: Determine the base URL of the page you are scraping. This is typically the URL of the page itself, without any additional paths or parameters.

2. Combine the base URL with the relative URL: Use the urljoin function from Python's urllib.parse module to combine the base URL with the relative URL, thus converting it to an absolute URL.

Here is an example in Python using lxml and urllib.parse:

from lxml import html
import requests
from urllib.parse import urljoin

# The URL of the page you want to scrape
base_url = 'http://example.com/some/page.html'

# Fetch the page
response = requests.get(base_url)
webpage = response.content

# Parse the webpage with lxml
tree = html.fromstring(webpage)

# Assume you're looking for all 'a' tags with 'href' attributes
for link in tree.xpath('//a/@href'):
    # Convert the relative URL to an absolute URL
    absolute_url = urljoin(base_url, link)
    print(absolute_url)

In this example, the urljoin function takes the base_url and a relative URL found in an href attribute of an a tag and resolves it to an absolute URL. This absolute URL can then be used to access the resource directly.

If you are using JavaScript with a library like cheerio on the server side (Node.js), you can similarly use the URL constructor to handle relative URLs:

const cheerio = require('cheerio');
const axios = require('axios');
const { URL } = require('url');

// The URL of the page you want to scrape
const baseUrl = new URL('http://example.com/some/page.html');

// Fetch the page using axios
axios.get(baseUrl.href).then(response => {
  const $ = cheerio.load(response.data);

  // Assume you're looking for all 'a' tags with 'href' attributes
  $('a').each((index, element) => {
    const relativeUrl = $(element).attr('href');

    // Convert the relative URL to an absolute URL
    const absoluteUrl = new URL(relativeUrl, baseUrl).href;
    console.log(absoluteUrl);
  });
});

In this JavaScript example, the URL constructor is used to resolve a relative URL against the baseUrl. The URL object takes two parameters: the relative URL and the base URL, and then href property of the resulting URL object gives you the absolute URL.

Remember to handle exceptions and edge cases, such as when a link is already an absolute URL or when it's a special kind of link (like a mailto: or javascript: link), which you might not want to convert. It's also a good practice to respect robots.txt and terms of service of websites when scraping.