Good coding practice for mixing JS and Python

Question

Say I have a Python script that takes a string and returns an int. But the string has to come from some JS function.

So what's the best way to go about calling this Python script from JS and having the script return the int back to JS? Assume I can't convert the Python code to JS code.

For example, if this was a .js file,

val = getString();
script = getPythonScript();
result = script.doPythonCode(val);