[Persistence-sig] Naive questions about getting and setting

[Donnal Walter]

1. Would naive (and rather application specific) questions such as
these be better posed to comp.lang.python? If so, I would happily
comply in the future.

2. In regard to the persistence API and especially in regard to
observation, would someone please point out the pitfalls of using
so-called "setter" and "getter" methods in attribute classes
themselves, as opposed to __setattribute__ and __getattribute__
methods in the container classes?

I am in no way proposing this as a general solution, but if in a
given situation one wanted to set up a scheme similar to that coded
below, what would be the major liabilities? Is it simply a matter
of lack of transparency? Or is there also a serious problem with
decreased efficiency?

=======
class Cell(object):
    def __init__(self, *args):
        """Arguments, if any, must be references to other cells."""
        self.__value = None         # the scalar value of the Cell
        self.__observers = []       # list of observers 

        if len(args) > 0:           # if this Cell is dependent
            self.ref = args           # save the list of references
            for i in self.ref:        # for every external ref
                i.AddObserver(self)     # register as an observer
            self.Update()             # set initial from refs
        else:                       # if this Cell is independent
            self.Reset(content)       # simply reset its value

    def AddObserver(self, observer):
        if observer not in self.__observers:
            self.__observers.append(observer)

    def _setValue(self, value):
        if value != self.__value:
            self.__value = value
            for o in self.__observers:
                o.Update()

    def _getValue(self):
        return self.__value

    def Set(self, input):
        try:
            # make sure input can be converted
            self._setValue(self.Encode(input))
        except ValueError:
            # if incompatible input value, reinitialize
            self.Reset()
        
    def Get(self):
        return self.Decode(self._getValue())

    def Encode(value):
        """ override to change Cell type"""
        return value

    def Decode(value):
        """ override to change Cell type"""
        return value
    
    def Update(self):
        """ Override in observer Cells.
        (Observers have access to the self.ref list.)
        """
        pass

    def Reset(self):
        """ may be overridden to change default value"""
        self._setValue('')
=======


=====
Donnal Walter
Arkansas Children's Hospital

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