I am trying to obtain a callable symbolic expression from a python script and having a hard time of it.

In my python script I would like to have something along the lines of:

```
func = myFunc()
func_evaluated_at_3 = myFunc(3)
func_dx = func.differentiate()
func_dx_evaluated_at_3 = func_dx(3)
```

In the module defining `myFunc`

I was hoping to have something along the lines of:

```
import sage.all as sage
class myFunc:
def __init__(self):
sage.var('x')
self.expression = x + 1
# maybe this should be: self.expression = sage.preparse("f(x) = x + 1")?
def __call__(self,value):
return self.expression(value)
def differentiate(self):
return self.expression.diff(x)
```

The goal is to keep the client code that constructs, calls, and manipulates objects of type myFunc from knowing about Sage. Perhaps I am misguided, any help would be appreciated. (also, help with the appropriate tags would be, well, helpful)