Get data back from Jython scripts using JSR-223

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.


<strong>UPDATE:</strong> 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


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


I had the same problem. I have written a scripting library that supports JSR223 and have submited http://bugs.jython.org/issue1798:

$ python Python 2.6.6 (r266:84292, Sep 15 2010, 16:22:56) [GCC 4.4.5] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> x=5<br/> >>> x<br/> 5 >>>

At least if you eval("x=5\nx") it seems like you would get the value back.


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