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

Question

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.

Answer 1

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().