----- "Alan Ristow" <alan.ris...@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 --
http://mail.python.org/mailman/listinfo/python-list
