Author here. I use Django and Rails as examples. Both are, indeed, "just a bunch of libraries". But many of those libraries, and especially their bundled colle

Author here. I use Django and Rails as examples. Both are, indeed, "just a bunch of libraries". But many of those libraries, and especially their bundled collection, enforce certain behaviour, shape and architecture. When you inherit from base-classes offered by a library, you are tightly coupling against that: i'd then stop calling it a library and call it a framework instead. The alternative would be a library that you inject, or compose. There's a big difference between (Rails) `class Project < ActiveRecord::Base; end` and class Project def initialize(project_repo) # Ruby has no interfaces, but this would be something that has at least an `.add(project)` @project_repo = project_repo end end The first inherits, couples tightly to a bunch of libraries (ActiveRecord::Base really is just a bunch of modules that you could pick and choose) the former injects behaviour which might come from libraries, or might come from your own code.