On Thu, Jan 22, 2009 at 3:04 PM, Marcus Goldfish <magoldf...@gmail.com> wrote:
> I'm trying to design a base class for a hierarchy. The properties I want to
> specify for the base class depend on the values of other properties of the
> base class. For instance, in this toy example of a base FoodProcessor
> class:
> class FoodProcessor:
> "Model for a kitchen appliance food processor"
> speed_settings # example [1, 2, 3, 4]
> blade_settings # example ["slice", "grate", "grind"]
> slice_thickness # example ["thin", "thick"], but only if
> blade_setting is "slice"
> it only make sense to have the slice_thickness property set when
> blade_settings = "slice"; otherwise, it would be preferable to only define
> the speed_settings and blade_settings properties. For example:
> class ProcessorA(FoodProcessor):
> "A slicing-only processor"
> speed_settings = [1, 2, 3, 4]
> blade_settings = "slice"
> slice_thickness = ["thin", "thick"]
> class ProcessorB(FoodProcessor):
> "A non-slicing processor"
> speed_settings = [1,2,3,4]
> blade_settings = ["grate", "grind"]
> slice_thickness = None
> Can anyone suggest some work-arounds, or refactoring for this type of
> design? Is there a design pattern for this?
I'm not sure what the problem is. You can provide default values in
the base class:
class FoodProcessor(object):
speed_settings = None
blade_settings = None
slice_thickness = None
Then in a derived class you can override just the ones you want to use:
class ProcessorB(FoodProcessor):
"A non-slicing processor"
speed_settings = [1,2,3,4]
blade_settings = ["grate", "grind"]
If you want to validate that the current settings make sense, you
could put a test in the __init__() method, which is simple to
implement but happens at instance creation time, or you could write a
metaclass to validate the settings at class creation time, which is
more difficult (to understand, anyway).
Kent
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