Best way to handle changing list of class imports

Scott Grant swrittenb at
Sun Oct 11 01:16:17 CEST 2009

On Oct 10, 6:44 pm, Dave Angel <da... at> wrote:
> Scott Grant wrote:
> > On Oct 10, 2:42 pm, "Diez B. Roggisch" <de... at> wrote:
> >> Scott Grant schrieb:
> >>> Hi there,
> >>> I'd like to set up a framework in which I can add or remove new
> >>> classes of a given expected subclass to my package, and have the
> >>> system load that set at runtime and be able to use them.  In essence,
> >>> if I have a class X, and subclasses A, B, and C that derive from X,
> >>> what's the most pythonic way to allow me to add or remove A, B, or C
> >>> from the package before runtime, to iterate over all known imported
> >>> classes that inherit from X, while minimizing the modifications to my
> >>> existing program.
> >>> Is the best way to set up a directory in my package for these classes,
> >>> and to define the list of active imports in the __all__ list in
> >>>  Or is there a better way?
> >>> For detail, I'm writing a system that students will be able to submit
> >>> game strategies that will compete against each other in a framework
> >>> that passes them game state and expects moves in return.  It would be
> >>> great to be able to add or remove these player strategies as new ones
> >>> come in, but I don't want to add a bunch of overhead importing each
> >>> one specifically in the game manager itself.
> >> I think you re-think your approach and kind of invert it. Instead of
> >> viewing your system as game-strategies plugged into the framework, make
> >> a game-strategy *use* your framework.
> >> Thus what your students deliver must be an executable script that sets
> >> up the game, and then passes the own strategy into it.
> >> Diez
> > Part of the benefit of the original approach (and the main reason I
> > want to use it that way) is that in multi-player or competitive games,
> > these strategies should be able to compete against each other.  It it
> > was a rock-paper-scissors system, I'd like strategy A to compete
> > against strategy B, and with large sets of strategies, to allow my
> > system to set up a crosstable with the minimal amount of hardcoding.
> > I'd like to know which strategy is best by setting up a competition
> > against all other submissions.
> I understand your goal to have several (or all) of the students'
> strategy files present, and competing.  But I have to ask whether you
> wouldn't be better off launching each student in a separate process (or
> separate machine, even), and competing over sockets or somesuch.  Two
> reasons come to mind:  1) students have a way of working *around* a
> problem, and sabotage (intended or not) is a definite possibility.  2)
> scalability to larger numbers of strategies.
> But I'll assume you've already considered and dismissed this kind of
> approach.  So essentially you're building an extensible framework, where
> parts of it are "discovered" at load time, and automatically merged in.

That's it, in a nutshell.  Instead of having some sort of scripting
language to author these strategies, it would be swell to just use the
native Python code.

Back in the day, I wrote a chess client that competed over the telnet
sessions at, and that was a really nice way to test out
the code.  However, the drawback was that I obviously couldn't get a
full crosstable of my code versus every other competitor on the
network.  In this case, I'd like to just set each opponent up against
every other, run n times, and see what the overall results are.  Doing
this locally (and additionally, allowing the competitors to test their
results locally for themselves) is a really exciting prospect.

> For that, I'd suggest reserving a directory at a known location, doing
> an os.path.dirname() on that directory, and building a list of module
> names.  Then use __import__() to load them, and build a list of module
> objects, and a list of classes in those modules.  Suggest classname to
> be simply formed by uppercasing the first letter of the module file.

Wonderful, I'll also take a look at this solution.  Thanks!

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