Ask HN: User authentication in web frameworks

I have worked with couple of web frameworks and have realised that user authentication is something which is common in most of the web apps. In my case, I ended up rewriting the user authentication module from scratch while developing a new web app. Is there any boilerplate code available for this specific task in different frameworks(for example flask, django), which is flexible enough to be used in different web apps? Or is there any library which makes this task easier? I would really like to see a repository containing example code for user authentication in different frameworks.