Confused about class relationships

[Tim Roberts]

John O'Hagan <research at johnohagan.com> 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, timr at probo.com
Providenza & Boekelheide, Inc.