urljoin

urljoin is a function in Python's urllib.parse module that constructs an absolute URL by combining a base URL with a possibly relative URL. It follows RFC 3986 resolution rules: if the second URL is absolute, it is returned unchanged; otherwise it is resolved against the base. The function also normalizes path segments and can remove or add components as needed to produce a valid URL.

How it resolves details: if the relative URL starts with '/', it replaces the base path after the domain. If it does not start with '/', it is joined to the directory portion of the base path; the base path's last segment is treated as a file unless the base ends with a slash. Dot segments such as '.' and '..' are resolved. If the relative URL contains a query, that replaces the base's query; if it contains a fragment, it becomes the resulting fragment.

Examples: urljoin('http://example.com/path/page.html', '../images/pic.jpg') yields 'http://example.com/images/pic.jpg'. urljoin('http://example.com/path/page.html', '/new') yields 'http://example.com/new'. If the relative URL is 'https://other.org/abs', the result is 'https://other.org/abs'.

Usage context: urljoin is widely used in web crawling, scraping, and link normalization to convert relative links found in pages into absolute URLs. The function is available in Python 3's urllib.parse; languages such as JavaScript provide similar URL resolution via the URL constructor or equivalent APIs.