Using eval with substitutions

a,b=3,4
x="a+b"
eval(x)
7
y="x+a"

Now I want to evaluate y by substituting for the evaluated value of x.
eval(y) will try to add "a+b" to 3 and return an error. I could do
this,
eval(y.replace("x",str(eval(x))))
10

but this becomes unwieldy if I have
w="y*b"
and so on, because the replacements have to be done in exactly the
right order. Is there a better way?

Thanks,
Abhishek

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