Jython - Correct way of creating class instances & executing methods from Python class

Question

I have a Python file which contains a class. I need to create an instance of that class and be able to call methods in that through Java.

I came up with a solution like this:

    PythonInterpreter r = new PythonInterpreter();
    r.execfile("File.py");


    PyObject p = r.eval("Car().begin()");
    System.out.println(p.toString());

And the python code:

class Car(SomeExtendedClass):

    myvar = 5

    def set(self, my):
        self.myvar = my;

    def begin(self):
        return self.myvar

Now, when I execute this, it prints 5 But if I run the following code:

    PyObject p = r.eval("Car().begin()");
    System.out.println(p.toString());

    r.eval("Car().set(7)");

    p = r.eval("Car().begin()");

    System.out.println(p.toString());

It will still print 5, instead of 7

It looks like I did not create one instance of Car and it always creating a new instance instead of using the created one.

Am I right?

Is it possible to create a new instance from a class in a Python file, and invoke/get data from methods with Java?

I have tried loading PyInstance using eval() but I get cannot cast exception from it:

return (PyInstance) this.interpreter.eval(className);

Artemkller545 · Accepted Answer · 2014-09-13 19:50:34Z

3

I just found out the solution for this "mystery"

At first, we want to execute the python file we're going to get instances from:

r.execfile("File.py");

And then define the PyObject which will contain the class that you want to invoke:

PyObject car = r.get("Car");

And then you have to call the __call__ method in order to create a new instance of Car, and cast it to PyObjectDerived:

PyObjectDerived p = (PyObjectDerived) o.__call__();

Now you can invoke methods, like this:

Python code:

def set(self, my):
    self.myvar = my;

Your java call:

p.invoke("set", Py.newInteger(5));

Hope I helped anyone.

answered Sep 13, 2014 at 19:50

Artemkller545

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Comments

langton · Accepted Answer · 2014-09-13 20:48:33Z

Whenever you call Car() you create a new instance of the class. So this create a new object and calls its set method:

r.eval("Car().set(7)");

But this then creates another instance rather than manipulating the existing one:

p = r.eval("Car().begin()");

The call to r.eval("Car().begin()"); creates a new python object as you require, but it actually returns a reference to the python object containing the return value from the begin() method - not to the instance created. This isn't what you wanted.

Leaving the python class exactly as defined, this code gets a reference to a single instance of the class:

PyObject p = r.eval("Car()");

(As you already saw, an alternative is to omit the parenthesis which gets you a reference to the python class object, and then using __call__ on it to create an instance).

Then to call the method on the existing instance:

p.invoke("set", Py.newInteger(7));

To get at the modified value, since it is an attribute of the instance and is not available via a 'getter' method, the getattr method of the PyObject class can get to it (or you could add a get method to the python code):

System.out.println(p.__getattr__("myvar"));

It's a shame that once you get a reference to the object via p you can't just call the method on it with java syntax, but of course Java knows nothing of the methods and/or attributes available on the Python object and even if it did they could change at any time because of the dynamic nature of Python. So you're left with methods like invoke to bind Java/Python together.

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