### Assign value to symbolic function?

I have a symbolic function $g(x,y)$, which depends on the variables $x$ and $y$. Using this, I define the function $f(x,y)$ as:
\begin{equation}
f(x,y) = g(x,y) + 2x.
\end{equation}

If I calculate the derivative of $f(x,y)$ with respect to $x$:
\begin{equation}
\frac{df(x,y)}{dx}=\frac{dg(x,y)}{dx}+2.
\end{equation}

Now, I need to evaluate this at $x=0$, knowing that $\frac{dg(x,y)}{dx}\bigg\rvert_{x=0}=10$. This should give me:
\begin{equation}
\frac{df(x,y)}{dx}\bigg\rvert_{x = 0} = \frac{dg(x,y)}{dx}\bigg\rvert_{x = 0} +2=12
\end{equation}

The code I have written to achieve this is the following:

```
x = var('x')
y = var('y')
g = function('g')(x,y) #symbolic function
f = g + 2*x
der_f = diff(f,x); der_f
```

and this is what I get:

```
diff(g(x, y), x) + 2
```

as I expected. However, I don't know how to follow. In particular, I need to know how to:

1) assign $\frac{dg(x,y)}{dx}\bigg\rvert_{x=0}=10$,

2) evaluate $\frac{df(x,y)}{dx}$ at $x=0$, so that I obtain $\frac{df(x,y)}{dx}\bigg\rvert_{x = 0} = 12$.