Keeping a variable constantly up-to-date

I'm certain this is a stupid question, [SPOILER]/prog/[SPOILER], but how does one keep a variable updated without doing it manually?

Example of what I'm looking for, preferably in pythong:

x = 10
y = x*2

x += 5

y == x*2
...
True

I could make a simple function to make sure var2 is always proportional to var1, but when I get to dozens or hundreds of variables, it gets to be a pain in the anus.

(function()
{ __defineGetter__('x', function(){ return _x; });
  __defineSetter__('x', function(v){ _x = v; y = v * 2; }); })();
x = 10;
alert(y);
x += 5;
alert(y);