# Evaluating a derivative of an unknown function

I need to evaluate the derivative of a certain unknown function, but instead of doing this what I get is the expression with a change in the name of the variable.

My example

```
z,l = var('z,l')
g = function('g', nargs=1)(z)
g_prime = g.derivative(z)(z)
g_prime(l)
```

So the output for that code is \frac{\partial}{\partial l} g (l) .

Instead, I would like \frac{\partial}{\partial z} g(l)

This does not quite do what you're asking, but is close: