How to sort a list of inter-linked tuples?

Question

lst = [(u'course', u'session'), (u'instructor', u'session'), (u'session', u'trainee'), (u'person', u'trainee'), (u'person', u'instructor'), (u'course', u'instructor')]

I've above list of tuple, I need to sort it with following logic.... each tuple's 2nd element is dependent on 1st element, e.g. (course, session) -> session is dependent on course and so on..

I want a sorted list based on priority of their dependency, less or independent object will come first so output should be like below,

lst = [course, person, instructor, session, trainee]

Answer 1

You're looking for what's called a topological sort. The wikipedia page shows the classic Kahn and depth-first-search algorithms for it; Python examples are here (a bit dated, but should still run fine), on pypi (stable and reusable -- you can also read the code online here) and here (Tarjan's algorithm, that kind-of also deals with cycles in the dependencies specified), just to name a few.