Question

Given a list of classes inheriting from this base:

class Plugin(object):
    run_after_plugins = ()
    run_before_plugins = ()

…and the following rules:

• Plugins can provide a list of plugins that they must run after.
• Plugins can provide a list of plugins that they must run before.
• The list of plugins may or may not contain all plugins that have been specified in ordering constraints.

Can anyone provide a nice clean algorithm for ordering a list of plugins? It will need to detect circular dependencies as well….

 def order_plugins(plugins):
      pass

I’ve come up with a few versions but nothing particuarlly neat: I’m sure some of you Art of Computer Programming types will relish the challenge 🙂

[note: question given in Python but it’s clearly not only a Python question: pseudocode in any language would do]

Answer 1

This is called topological sorting.

The canonical application of
topological sorting (topological
order) is in scheduling a sequence of
jobs or tasks; topological sorting
algorithms were first studied in the
early 1960s in the context of the PERT
technique for scheduling in project
management (Jarnagin 1960). The jobs
are represented by vertices, and there
is an edge from x to y if job x must
be completed before job y can be
started (for example, when washing
clothes, the washing machine must
finish before we put the clothes to
dry). Then, a topological sort gives
an order in which to perform the jobs.