Why is a variable not updating after changing its dependent variable? [duplicate]

Question

I don't understand why the variable 'y' doesn't update when I change the x? (The 'y' variable is dependent on 'x' right?)

x = 5
y = x*2

print(x)
print(y)

x = 3

# Expect it to print '3' and '6' instead it print '3' and '10'
print(x)
print(y)

Answer 1

(The 'y' variable is dependent on 'x' right?

No.

Few programming languages have dependent / computed variables^[0] and Python is not one of them^[1]. When y = x*2 is executed, the expression on the right-side of the = is fully evaluated and the result set as the value of y. y is thereafter independent from x^[2].

Generally speaking, if you want y to be a function of x... you define it as a function of x:

x = 5
def y(): return x*2

print(x)
print(y())

x = 3

# Expect it to print '3' and '6' instead it print '3' and '10'
print(x)
print(y())

---

0. I know of make's lazy variables and Perl's tied scalars
1. it does have computed attributes (aka properties) but that's a very different thing
2. There are situations which kind-of look like dependent variables e.g. if you set y to a mutable sub-structure of x changes to this sub-part of x will be visible through y. That's not actually a dependency though, it's just that the two variables point to the same (mutable) structure, so both "see" mutations applied to that shared structure.