How to sort a list of interconnected 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 have a list of tuples above, I need to sort it with the following logic .... each element of the 2nd tuple depends on the 1st element, for example. (course, session) → the session depends on the course, etc.

I want the sorted list to be based on the priority of their dependency, the first or less object will be the first, so the output should look like this:

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

Answer 1

, . wikipedia Kahn ; Python ( , ), pypi ( ), ) ( , ), .

Answer 2

, , , , . Python ( , , ), , http://www.bitformation.com/art/python_toposort.html

- , items , partial_order. lst . , itertools.chain.from_iterable(lst),

import itertools
lst = ...
ordering = topological_sort(itertools.chain.from_iterable(lst), lst)

, , lst.

EDIT. topsort Alex Martelli, lst .