John O'Hagan <[EMAIL PROTECTED]> wrote: > >Apologies if this is a D.Q., I'm still learning to use classes, and this >little problem has proved too specific to find in the tutorials. > >I have two classes with a relationship that I find confusing. > >One is called Engine, and it has a method (bar_builder) which generates >instances of the other class, called Bar (not as in Foo but as in bar of >music; inherits from list). > >Also, Bar takes the generating instance of Engine as an argument to its >__init__ method: > >class Bar(list): > > def __init__(self, a_bar, args, engine): > list.__init__ (self, a_bar) > self[:] = a_bar > self.args = args > self.engine = engine > #more instance attributes... > > #methods... > >class Engine: > > def __init__(self, args): > self.args = args > #more instance attributes... > > def bar_builder(self): > #body of method generates lists... > yield Bar([generated_list], args, self) > > #more methods... > >#(other stuff...) > >def main(args): > > engine = Engine(args) > bars = engine.bar_builder() > for a_bar in bars: > #play the music!... > >While this works (to my surprise!) and solves the problem which motivated it >(i.e. Engine instances need to pass some attributes to Bar instances ), it >seems too convoluted. Should one class inherit the other? If so, which way >around? Or is it fine as is?
You need to put on a philosophical hat for this. Inheritance is used for an "is-a" relationship, where one thing "is-a" special type of another thing. Is "Engine" a special type of "Bar"? Is "Bar" a generic type of "Engine"? I think the answer is no, and because of that inheritance is not the right answer. >I'm hoping this is a common trap I've fallen into; I just haven't been able to >get my head around it. (I'm a musician...) Why do you think this is a trap? Engine creates Bars. Bars need to know about the Engine. I think you have expressed that relationship properly. When I have these kinds of philosophical debates over my own code, I always try to summarize them in the comments inside the module, just so others can get "inside my head" to understand the thinking that led to my design. -- Tim Roberts, [EMAIL PROTECTED] Providenza & Boekelheide, Inc. -- http://mail.python.org/mailman/listinfo/python-list
