# substitute variable inside function

I can do

```
f(a,b) = a^2+b^3; f=f-f.subs(a=(a-1))
(a, b) |--> -(a - 1)^2 + a^2
```

I tried to write a function which does this for arbitrary expressions and variables

```
def test(self, symb):
return (self-self.subs(symb=(symb-1)))
```

but

```
test(f,a)
(a, b) |--> 0
```

fails miserably. It seems self.subs(symb=(symb-1)) can not be evaluated properly. How can I achieve what I want?