[Python-Dev] PEP 318: Properties (original) (raw)

Jewett, Jim J jim.jewett at eds.com
Sun Apr 4 21:07:32 EDT 2004 

Skip> x = 42

Skip> def x(self) [propget]:
Skip>   "Doc string for x"
Skip>   return self.__x

Guido> It's broken.  I expect this to raise an exception at some point.
Guido> Beyond that, who cares?

Jim> If so, then this decorator is a bad idea.

Jim> I would expect 42 to be the initial value of the property x.

Skip> Then it makes sense to put "x = 42" after the three property 
Skip> definitions (before the end of the class definition).  
Skip> Would that work?

Not really. The documentation says 

"Properties are a neat way to implement attributes whose usage 
resembles attribute access, but whose implementation uses method 
calls."

This suggests that creating a property creates a manager. In this case, the manager is created for an attribute that already exists. If it uses the previous value as the initial current value, that is what I expect.
If it raises an exception ("I can't manage that! Someone beat me to it!"), I'll be annoyed, but live with the limitation. If it silently throws the value away, or raises an exception at run time -- that is bad.

Yes, this is a variation on the "Properties do not work for classic classes, but you don't get a clear error when you try this." wart.
But the code this decorator replaces::

x = property(getx, setx)

does explicitly replace x, so I think this confusion may be specific to the decorator.

-jJ

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