How can I substitute the value of one variable in a multivariate function?

Suppose, I have a function f(x,y)=x²+y². Now, I would like to substitute x=2, keeping y symbolic. I tried with

f.evalf(subs={x:2})

But it is not substituting the value of x, and just giving me the original expression.

You didn't resolve all the symbols, so you won't get an evaluation which is what evalf tries to do - it only works if it substitutes all the variables in your expression.

You can however use subs directly on your expression.

>>> from sympy import *

>>> x, y = symbols('x y')

>>> expr = x**2 + y**2

>>> expr.subs(x, 2)
y**2 + 4

And if needed, could subsequently call evalf().