Re: accessor/mutator functions

[Michael Spencer]

[EMAIL PROTECTED] wrote:
If the class had two attributes--x and y--would the code look like
something lik this:
 class C(object):
        def __init__(self):
            self.__x = 0
            self.__y = 0
        def getx(self):
            return self.__x
        def setx(self, x):
            if x < 0: x = 0
            self.__x = x
        def gety(self):
            return self.__y
        def sety(self, y):
            if y < 0: y = 0
            self.__y = y
        x = property(getx, setx)
        y = property(gety, sety)
It could do - that works.  One feature of this solution is that it leaves the 
accessor/mutator functions in the namespace.  That may be a good or a bad thing. 
 If bad, you could simply delete them after the property call (which is 
probably better written as close as possible to the functions)

i.e., class C(object):
   def __init__(self):
       self.__x = 0
       self.__y = 0
   def getx(self):
       return self.__x
   def setx(self, x):
       if x < 0: x = 0
       self.__x = x
   x = property(getx, setx)
   del getx, setx
   def gety(self):
       return self.__y
   def sety(self, y):
       if y < 0: y = 0
       self.__y = y
   y = property(gety, sety)
   del gety, sety
There are also recipes in the cookbook for defining property "suites" more 
elegantly
Note, that it is also easy to "roll your own" descriptor, which may be 
worthwhile if you have a lot of similar properties, for example (not tested 
beyond what you see):

from weakref import WeakKeyDictionary
class Property(object):
    def __init__(self, adapter):
        """adapter is a single argument function that will be
            applied to the value before setting it"""
        self.objdict = WeakKeyDictionary()
        self.adapter = adapter
    def __get__(self, obj, cls):
        if isinstance(obj, cls):
            return self.objdict[obj]
        else:
            return self
    def __set__(self, obj, value):
        self.objdict[obj] = self.adapter(value)
class C(object):
    x = Property(lambda val: max(val, 0))
    y = Property(lambda val: val%2)
    z = Property(abs)
 >>> c= C()
 >>> c.x = -3
 >>> c.x
0
 >>> c.y = -3
 >>> c.y
1
 >>> c.z = -3
 >>> c.z
3
 >>>
Michael
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