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.

>>22
Whether or not that's what it's "for", it gets the job done and it's much simpler (and less line-consuming) than

x = 2
def y():
    x*2
every single time I need a new variable.