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.

>>23
python already lets you stick it all on one line
$ cat anus.py
def foo(): print "hax my anus"
def bar(): print "fuck my anus"; foo()
bar()
$ python anus.py
fuck my anus
hax my anus