jython-users

[Jython-users] subclassing problems

[Erlend V <eb...@im...>]

hi,

I'm having some problems with the following situation:

I have a Java class that I extend in Jython.
When I try to use this Jython class from Java, I get an exception similar to 
the following: "TypeError: instance already instantiated for 
problem.JavaClass"

What am I doing wrong?

Erlend
-----

Here is the console output and test programs:

[evb@evb unittest]$ jythonc -p problem problem/PyClass.py 
.... stuff deleted......

[evb@evb unittest]$ java problem.UsesPyClass
JavaClass called with 'hi from Java'
Exception in thread "main" Java Traceback:
 
        at java.lang.Throwable.<init>(Throwable.java:84)
        at java.lang.Exception.<init>(Exception.java:35)
        at java.lang.RuntimeException.<init>(RuntimeException.java:39)
        at org.python.core.PyException.<init>(PyException.java:44)
        at org.python.core.PyException.<init>(PyException.java:56)
        at org.python.core.Py.TypeError(Py.java:120)
        at 
org.python.core.PyReflectedConstructor.__call__(PyReflectedConstructor.java:88)
        at 
org.python.core.PyReflectedConstructor.__call__(PyReflectedConstructor.java:72)
        at org.python.core.PyInstance.__init__(PyInstance.java:167)
        at org.python.core.Py.initProxy(Py.java:746)
        at problem.PyClass.<init>(PyClass.java:100)
        at problem.UsesPyClass.main(UsesPyClass.java:6)
Traceback (innermost last):
  (no code object) at line 0
TypeError: instance already instantiated for problem.JavaClass

The Java class that the jython class extends: 
------
package problem;

public class JavaClass{

  public JavaClass(String name) {
    System.out.println("JavaClass called with '" + name + "'");
  }
}
-----
The Jython class that extends this java class:
-----
import problem

class PyClass(problem.JavaClass):
    def myMethod(self):
        "@sig void myMethod()"
        print "python method myMethod called"

if __name__ == "__main__":
    x = PyClass("hi from python")
    x.myMethod()
-----
The Java class that uses the PyClass : 
-----
package problem;

public class UsesPyClass {

  public static void main(String[] args) {
    PyClass py = new PyClass("hi from Java");
    py.myMethod();
  }
}

Re: [Jython-users] subclassing problems

[<bc...@wo...>]

[Erlend V Bøe]

>I'm having some problems with the following situation:
>
>I have a Java class that I extend in Jython.
>When I try to use this Jython class from Java, I get an exception similar to 
>the following: "TypeError: instance already instantiated for 
>problem.JavaClass"
>
>What am I doing wrong?

You need a python constructor in your PyClass, and when using jythonc,
the constructor must *not* call the superclass ctor. Something along the
lines of:

import problem

_jythonc = 1

class PyClass(problem.JavaClass):
    def __init__(self, s=None):
        if not _jythonc:
            problem.JavaClass.__init__(self, s)

    def myMethod(self):
        "@sig void myMethod()"
        print "python method myMethod called"

if __name__ == "__main__":
    _jythonc = 0
    x = PyClass("hi from python")
    x.myMethod()




In the generated PyClass.java, we have code like:

public class PyClass extends problem.JavaClass 
    implements org.python.core.PyProxy
{
    public PyClass(java.lang.String arg0) {
        super(arg0);
        Py.initProxy(this, ... new Object[] {arg0}, ...);
    }
}

i.e, the JavaClass ctor is called before any python code gets to
execute. Without the __init__() method, jython will automaticly call the
JavaClass ctor (again) and the exception "instance already instantiated"
is thrown.

regards,
finn