Get data back from Jython scripts using JSR-223

Question

Hello. I am using Jython 2.5.1 with JSR-223 (i.e. javax.script package) and I expect the last line of the Python script to be returned. For example, after evaluating this script:

class Multiplier:

  def multiply(self, x, y):
    return x * y

Multiplier().multiply(5, 7)

I should get back 35, but I get null instead. In other hand it works with this other test:

5 * 7

What am I doing wrong?

Here's the Java code:

public static void main(String[] args) throws Exception {
    ScriptEngine engine = new ScriptEngineManager().getEngineByName("python");

    FileReader f = new FileReader("Multiplier.py");
    Object result = engine.eval(f);
    //assert(result == 35);
}

PS: It works fine with JRuby, Groovy and Rhino, i.e. the last line is always returned.

Thanks in advance.

Answer 1

UPDATE: I was actually missing the goal (and problem) of the OP in my initial answer that has been clarified in a comment. I'm updating my answer accordingly.

First update the Multiplier.py script as below:

class Multiplier:

  def multiply(self, x, y):
    return x * y

x = Multiplier().multiply(5, 7)

Then call it like this from the Java code:

public static void main(String[] args) throws Exception {
    ScriptEngine engine = new ScriptEngineManager().getEngineByName("python");

    FileReader f = new FileReader("Multiplier.py");
    engine.eval(f);
    Object x = engine.get("x");
    System.out.println("x: " + x);
}

I get the following output when running the code above:

x: 35

Answer 2

This is a Python language issue more than a Jython or JSR 223 issue. Python differentiates between expressions (which have values) and statements (which don't). The script you're passing is a statement. If you passed an expression, it'd have a value.

The reason you're seeing something different with Ruby and JavaScript is that compound statements have the value of the last statement evaluated. For example, compare Ruby:

>> (2 ; 3) + 5
=> 8
>> (x = 5) + 7
=> 12

with Python:

>>> (2 ; 3) + 5
  File "<stdin>", line 1
    (2 ; 3) + 5
       ^
SyntaxError: invalid syntax
>>> (x = 5) + 7
  File "<stdin>", line 1
    (x = 5) + 7
       ^
SyntaxError: invalid syntax

JavaScript seems to be somewhere in between. Like Ruby, assignments evaluate to the value assigned. However, the last evaluated statement in a block is returned but not usable as part of an expression:

> { 2 ; 3 }
3
> { 2 ; 3 } + 5
5
> (x = 5) + 7
12