Sympy "global" substitution

Question

I have a number of symbolic expressions in sympy, and I may come to realize that one of the coefficients is zero. I would think, perhaps because I am used to mathematica, that the following makes sense:

from sympy import Symbol
x = Symbol('x')
y = Symbol('y')
f = x + y
x = 0
f

Surprisingly, what is returned is x + y. Is there any way, aside from explicitly calling "subs" on every equation, for f to return just y?

Answer 1

I think subs is the only way to do this. It looks like a sympy expression is something unto itself. It does not reference the pieces that made it up. That is f only has the expression x+y, but doesn't know it has any link back to the python objects x and y. Consider the code below:

from sympy import Symbol
x = Symbol('x')
y = Symbol('y')
z = Symbol('z')

f1 = x + y
f2 = z + f1
f1 = f1.subs(x,0)
print(f1)
print(f2)

The output from this is

y
x + y + z

So even though f1 has changed f2 hasn't. To my knowledge subs is the only way to get done what you want.