How To capture an id into an array..

Christian Tismer tismer at
Mon Mar 29 19:45:46 CEST 2004

Balaji wrote:

> Thanks for all the initial help....
> Sorry for bugging you again...
> Here is a part of my code..
> Suppose if some body wants to assign an e=E() to an expression class..
> he may define then e=v+v1(where v and v1 both belong to a variable class)
> now again if we define say v2=V() and then try to multiply v2 with e then I want
> to raise an error that it is a nonlinear term..
> So  what I though was if i keep track of all the instances id then I might raise
> an exception...
> But the problem was i couldnt check the right side of an assigment...
> Can the right side of an assignment be checked...

You can intercept the process of assignment.
In older python, this was done overwriting __setattr__,
but today it is much simpler and cleaner to user property
objects. You can just define a setter that does certain checks.

 >>> print property.__doc__
property(fget=None, fset=None, fdel=None, doc=None) -> property attribute

fget is a function to be used for getting an attribute value, and likewise
fset is a function for setting, and fdel a function for del'ing, an
attribute.  Typical use is to define a managed attribute x:
class C(object):
     def getx(self): return self.__x
     def setx(self, value): self.__x = value
     def delx(self): del self.__x
     x = property(getx, setx, delx, "I'm the 'x' property.")

ciao - chris

Christian Tismer             :^)   <mailto:tismer at>
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