Edit: plus, if I'm plotting with Sympy it's almost certainly something I've calculated symbolically with Sympy, so everything is defined and ready, and it's much more elegant than switching to a separate program. For a one-off "what does function look like?" question, both these tools are far too clunky, and I'd use Desmos or GeoGebra.
Sympy is just a Python library, and one of the types it defines is symbolic maths variables, so you define Python variables just like with any other library, and then use them.
And this, my friends, is why mathematicians will never feel at home using Python+libraries. Instead of having a programming environment ideally suited to the needs of math, you need to fit your math to the Procrustean bed of Python syntax.
You just want to plot the sine function, but first you have to import libraries, declare symbols (with a scary correspondence between strings and variable names), and finally plot the damn function.