Re: reuse validation logic with descriptors

[Michael Spencer]

David S. wrote:
This still fails to work for instances variables of the class.  That is 
if I use your property in the following:
py> ...class Flags(object):
...        def __init__(self): 
...             a = singlechar
...
you should write that as:
class Flags(object):
    a = singlechar
    def __init__(self):
        a = "a"

py> f = Flags()
py> f.a = "a"
Now f.a.__class__.__name__ returns 'str'.  So the property was not 
used at all.

Also, it seems that using a property, I can not do the other useful 
things I can do with a proper class, like provide an __init__, __str__, 
or __repr__.  

If you want "other useful things" then you can write a custom descriptor, like:
from weakref import WeakKeyDictionary
class SingleChar(object):
    def __init__(self):
        """raises ValueError if attribute is set to something
        other than a single char"""
        self.objdict = WeakKeyDictionary()
    def __get__(self, obj, cls):
        if isinstance(obj, cls):
            try:
                return self.objdict[obj]
            except KeyError:
                raise AttributeError, "property not set"
        else:
            return self
    def __set__(self, obj, value):
        if isinstance(value, str) and len(value) == 1:
            self.objdict[obj] = value
        else:
            raise ValueError, value
class Flags(object):
    a = SingleChar()
    b = SingleChar()
See also: 
http://groups-beta.google.com/group/comp.lang.python/msg/30c61a30a90133d2
for another example of this approach
Michael
Again, thanks,
David S.

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