Using property in classic class may not work

[Alex Martelli]

Neil Hodgson wrote:

>     The "property" feature in Python 2.2 appears very helpful to me and
> seems to work on both classic and new-style classes. However, pychecker
> gives this error message:
> 
> DemonTerm.py:563: Using property (frame) in classic class DemonFrame may
> not work
> 
>     The class in question is derived from a wxWindows class, so I can't
> make it derive just from object. The "What's New in Python 2.2" document
> only mentions property wrt new-style clases. Can I depend on property
> working for classic classes? Is there a workaround such as deriving from
> object as well?

No, and yes, respectively -- deriving from object means your class
isn't classic any more, so properties do work as intended there.  Vide:

>>> class X:
...   def __init__(self, x=23): self.xx=x
...   def getx(self): return 1000+self.xx
...   def setx(self,x): self.xx=x
...   x=property(getx,setx)
...
>>> x=X()
>>> x.x
1023
>>> x.x=99
>>> x.x
99

The assignment hasn't been caught by setx, so x.xx is still
23 and x.x is now the integer 99, not a property any more.  But:

>>> class Xob(X,object): pass
...
>>> x=Xob()
>>> x.x
1023
>>> x.x=99
>>> x.x
1099
>>>

as you see, this does work as intended.


Alex