Jackson wrote: > Thanks for the reply. > > danielx wrote the following on 2006-08-13 19:49: > > Is your declaration of ABC supposed to have some_super as one of the > > base classes? Your constructor has some_super as a parameter. What is > > this supposed to mean in light of the declaration for ABC? > > Indeed, my goal is to have the base class of ABC determined dynamically > via a parameter passed into the constructor. > > > > If you are trying to customize the base class of ABC by passing an > > argument to the constructor of ABC, you should probably reconsider. If > > constructing one instance of ABC can change ABC (the class) itself, > > then the behavior of other instances will be affected as well. No > > programmer can stay sane if he creates instances of a class that could > > suddenly change its behavior (due to someone else's code). > > Fortunately, the ABC class is not very useful. In fact, it was mostly > just to be used to store particular examples of the user-specified base > class. So all method calls would be from the base class only. > > > > > What you could do instead is to create a function which constructs > > classes based on the arguments it recieves. Then, you'll be able to > > create instances of the generated classes (all this meta-thinking is > > huring my brain ;). I am talking about something like this: > > > > def createClass(name, base): > > exec "class %s(%s): pass" % (name, base) > > return eval( "name" ) > > In fact, this is exactly what I ended up doing. > > def example(base): > if base == SomeClass: > # call SomeClass.__init__(self) > # build example as it would look in SomeClass > > elif base == SomeOtherClass: > # call SomeOtherClass.__init__(self) > # build example as it would in SomeOtherClass > > > Can you please tell us why you are doing this? My curiosity is killing > > me! > > > > So here is a good example: > > I have 4 classes: > > Lion(Animal): > Ant(Animal): > Bee(Animal): > Human(Animal): > > which are all subclasses of some superclass called Animal. Now I want > to define an occupation. For example, Worker. A worker can exist as any > of the 4 classes above. Their constructors are different and I might > want to add certain features. > > My first thought was to create a class called "Worker" and have the base > class determined by a variable which is passed into the constructor.
What you should keep in mind though, is the "is-a" relationship that subclasses are supposed to have with their parents. "worker is a lion" does not make sense. What you want instead is LionWorker. You'll end up doing only a little more typing than if you had a general Worker class, but I don't think it will be much more. If your *Worker classes have really simple bodies, you might be able to automate the creation of your *Worker classes by doing something like the createClass function (ie, by creating the classes dynamically instead of using the class statement). I also discovered another way to dynamically create classes, described in this article (the article talks about other things as well): http://www.onlamp.com/pub/a/python/2003/04/17/metaclasses.html I'm not sure if this will help you, but it's definitely interesting. > Most of the method calls will come from the Animal superclass anyway, > but some method calls might come from the Lion class, for example. > > Now I realize this would drive a programmer crazy...because a Lion might > have a roar() method whereas a Human might have a holler() method. But > so long as the user knew which argument they passed in, it shouldn't be > too difficult to keep track of it. > > So for example (again, I know what I am typing doesn't actually work)... > > Worker(some_animal): > def __init__(self,some_animal): > # change the base class to some_animal > > if some_animal == Lion: > # give the lion a big mane > > if some_animal == Ant: > # make the ant dumb > > if some_animal == Bee: > # make the bee know how to dance > > if some_animal == Human > # give the human a hardhat > > def work(self, hours): > # tell the animal to work for the specified number of hours > if some_animal == Lion: > self.cat_nap(hours) > if some_animal == Ant: > self.walk_back_and_forth(hours) > if some_animal == Bee: > self.buzz_and_dance(hours) > if some_animal == Human: > self.use_hammer_on_coworker(hours) > # notice, a Human does not have a cat_nap method > > def take_lunch(location): > > .... > > and so on. So the thought is that a Worker can exist in many different > forms: as a lion, as an ant, as a bee, and as a human. And I might want > a single worker class that represents them all. > > Hopefully this makes sense. > > > Another meta-thought: Hopefully I've beaten everyone else to the punch > > about that question. Is it just me, or will a reply with such a > > question always tell the original poster that what he wants to do MUST > > be flawed? I hope I have been gentler than this. > > > > :-) There is no need to be too gentle. We are all here to learn (or > help). So I am fairly happy with the def solution...any comments on True, but that doesn't mean you have to stand a rude response. I'm just hoping for a little more courtesy than what I've seen. > this? But a Worker is an noun, and it seems like the proper way to do > this is to make the Worker into a class...so that I can define methods > like "work", "take_lunch", etc. However, I have no idea how I should do > this. Perhaps someone can recommend a procedure. > > thanks. -- http://mail.python.org/mailman/listinfo/python-list
