How to call Python Methods from Mathematica employing ExternalEvaluate?

Question

This question is about the compatibility of methods written in Python and ExternalEvaluate from Mathematica.

ExternalEvaluate allows Python functions to be called from Mathematica. For example, the function "surfaceArea" from sphere.py:

from math import pi
def surfaceArea(r):
    return 4*pi*r**2

may be called from Mathematica in the following way

pySession = StartExternalSession["Python"];
ExternalEvaluate[pySession, File["sphere.py"]];
radius = 2.0;
ExternalEvaluate[pySession, "surfaceArea" -> {radius}]

Now, consider the sphere Python class (sphereClass.py)

from math import pi
class sphere(object):

    def __init__(self,radius):
        self.radius = radius

    def volume(self):
        return (4/3)*pi*self.radius**3

Aim: Evaluate volume from sphereClass.py at radius = 2.0 employing ExternalEvaluate.

Answer 1

There are many ways to send Mathematica data to python, here you'll see direct string manipulation:

Initial code

pySession = StartExternalSession["Python"];
ExternalEvaluate[pySession, "from math import pi
class sphere(object):

    def __init__(self,radius):
        self.radius = radius

    def volume(self):
        return (4/3)*pi*self.radius**3"];

radius = 2.0;

Passing data

ExternalEvaluate[pySession, 
 "sp1=sphere(" <> ExportString[radius, "PythonExpression"] <> 
  ");sp1.volume()"]

(*Out: 33.5103*)

Or you can make it more readable by using StringForm:

ExternalEvaluate[pySession, 
 ToString@StringForm["sp2=sphere(``);sp2.volume()", 
   ExportString[radius, "PythonExpression"]]]

(*Out: 33.5103*)

finaly

Stop the session:

DeleteObject[pySession]