### Prevent sage from calculating a function?

So I have some code where I would like to formally plug in a number value into a symbolic function without having sage calculate it.

For example, if I have;

```
var(x)
f(x) = x^2
c = 2
```

And then I put in f(c), I'd like it to return 2^2 and not 4. I imagine there must be a ~~while ~~way to do this, but I can't seem to find it with googling. Any suggestions would be appreciated.