parsing a Python expression from Java

I've got a bit of an interesting challenge

To the point:

I want to allow a user to enter an expression in a text field, and have that string treated as a python expression. There are a number of local variables I would like to make available to this expression.

I do have a solution though it will be cumbersome to implement. I was thinking of keeping a Python class source file, with a function that has a single %s in it. When the user enters his expression, we simply do a string format, and then call Jython's interpreter, to spit out something we can execute. There would have to be a number of variable declaration statements in front of that expression to make sure the variables we want to expose to the user for his expression.

So the user would be presented with a text field, he would enter

x1 + (3.5*x2) ** x3

and we would do our interpreting process to come up with an open delegate object. We then punch the values into this object from a map, and call execute, to get the result of the expression.

Any objections to using Jython, or should I be doing something other than modifying source code? I would like to think that some kind of mutable object akin to C#'s Expression object, where we could do something like

PythonExpression expr = new PythonExpression(userSuppliedText)
expr.setDefaultNamespace();
expr.loadLibraries("numPy", /*other libraries?*/);

//comes from somewhere else in the flow, but effectively we get
Map<String, Double> symbolValuesByName = new HashMap<>(){{
    put("x1", 3.0);
    put("x2", 20.0);
    put("x3", 2.0);
}};

expr.loadSymbols(symbolValuesByName);

Runnable exprDelegate = expr.compile();

//sometime later
exprDelegate.run();

but, I'm hoping for a lot, and it looks like Jython is as good as it gets. Still, modifying source files and then passing them to an interpreter seems really heavy-handed.

Does that sound like a good approach? Do you guys have any other libraries you'd suggest?

Update: NumPy does not work with Jython

I should've discovered this one on my own.

So now my question shifts: Is there any way that from a single JVM process instance (meaning, without ever having to fork) I can compile and run some Python code?