[Python-Dev] read-only properties

John Williams jrw at pobox.com
Tue Oct 8 22:45:15 CEST 2002

Guido van Rossum wrote:
>>The key seems to be that property() doesn't know the name the
>>property has.
> Oops, you're right.  This makes it practically impossible to improve
> the error message (without the kind of trick that Jeff shows).
> --Guido van Rossum (home page: http://www.python.org/~guido/)
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Why not have the property find its name in the class's __dict__, 
something like this:

(in class property)
   def __set__(self, obj, value):
     if self.fset is not None:
       self.fset(obj, value)
       for key, value in type(obj).__dict__.iteritems():
         if value is self:
           raise AttributeError, "Property '" + key + "' is read-only."
       # No name?  Strange put possible I guess.
       raise AttributeError, "can't set property"

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