Advice requested on class design

Jean-Michel Pichavant jeanmichel at sequans.com
Wed Apr 28 15:16:51 CEST 2010


----- "Alan Ristow" <alan.ristow at gmail.com> wrote: 
> Hi all, 
> 
> I am relatively new to Python, though not to programming in general, and using Python 2.6. I have a design problem that I cannot quite decide how to handle and I am hoping for some advice. 
> 
> I would like to have three classes, ClassA, ClassB, and ClassC, that are essentially the same, the only difference being that each class has a different range of valid values for its properties. Thus, the obvious solution is to create a base class, then subclass from that and include the required error-checking during initialization and when property values change. The base class might look something like this: 
> 
> class BaseClass(object): 
> def __init__(self, arg1, arg2): 
> self.a = arg1 
> self.b = arg2 
> 
> @property 
> def a(self): 
> return self.a 
> 
> @a.setter 
> def a(self, new_a): 
> self.a = new_a 
> 
> @property 
> def b(self): 
> return b 
> 
> @b.setter 
> def b(self, new_b): 
> self.b = new_b 
> 
> And ClassA might look something like this: 
> 
> class ClassA(BaseClass): 
> def __init__(self, arg1, arg2): 
> assert arg1 > 0 and arg2 > 100 
> BaseClass.__init__(self, arg1, arg2) 
> 
> @a.setter 
> def a(self, new_a): 
> assert new_a > 0 
> self.a = new_a 
> 
> @b.setter 
> def b(self, new_b): 
> assert new_b < 100 
> self.b = new_b 
> 
> However, instead of rewriting my validation code in several different places, I would prefer to write it one time and keep it in one place. I can write a function or method that I call each time I need to do validation, which is the approach I would take in most languages. However, since I am basically writing several small variations on one theme, it seems like in Python this might be an ideal application for decorators and/or metaclasses. 
> 
> So my question is, what is the most sensible way to write a set of classes such as these in Python? I am not afraid to experiment with decorators or metaclasses -- indeed, I would love to learn more about them -- but in this particular project I do not want to use them just for the sake of learning to use them. If there is a good reason to use them, I am all for it, though. 
> 
> All advice appreciated, 
> 
> Thanks, 
> 
> Alan 
> 


I will try not to get too religious :o) 
Validate your args in the base class. 


class Base: 
MIN = None 
MAX=None 
self.__init__(self, arg1, arg2): 
# don't use asserts, they are removed in optimized code 
if self.MIN is None or self.MAX is None: 
raise TypeError('Abstract class') 
if arg1 < self.MIN or arg2 > self.MAX or arg2 < arg1: 
raise ValueError('arg1 arg2 not in range [%s, %s]' % (self.MIN, self.MAX)) 
self.a = arg1 
self.b = arg2 


class ClassA(Base): 
MIN = 5 
MAX = 12 


class ClassB(Base): 
MIN = 8 
MAX = 25165 




JM 
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