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Properties - when and why

Arthur Siegel ajs at ix.netcom.com
Sun Jul 7 19:48:11 EDT 2002 
Learning something about properties - as usual the hard way.
Beginning to conclude that they, as I recently learned with __slots__,
are more "special case" than I first understood.

Say self.W and self.U are each 3 element vectors referenced on
initialization, with attributes x,y, z. V is a calculated vector.  V=W-U

def get_V(self):
   return self.W-self.U

V=properties(get_V)

But occasionally I need to access V's attributes individually.

def V_homogenous(self):
    return array((V.x,V.y,V.z,1.))

Have I not just made three calls to the vector subtraction
of the get_V method? Not good.

Could have done:
def get_V(self):
   self._V  = self.W - self.V
   return self._V

def V_homogenous(self):
    return array((self._V.x,self._V.y,self._V.z,1.))

But now the use of the properties facility seems superfluous and
redundant. As a practical matter its self._V, the old style attribute
I will be referencing, not self.V, the new style property.

Is the when and why of using properties something that can be
explained in few enough words?

Art
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