[Py4j-users] Interface troubles

[Alec C. <ar...@fa...>]

Hi,
I've just started trying out py4j and am finding it really helpful so far.  I
was able to implement a simple Java interface in Python, but I'm running into
trouble with more complicated interfaces.

Specifically, I'm having trouble when one interface has a function that returns
another interface, with both interfaces implemented in Python.  The problem
seems to be that the python object is only treated as a Java interface when it
is passed as an argument, not when it is the return value.

I've included a simple example below.  Is what I'm trying to do possible? 
Thanks for any help,
Alec

Here's the python code:
--------------------------------------------------

from py4j.java_gateway import JavaGateway

class A(object):
    class Java:
        implements = ['py4j.examples.InterfaceA']

class B(object):
    def foo(self):
        return A()                                   #this doesn't work
        return gateway.jvm.py4j.examples.ClassA()    #this does
    class Java:
        implements = ['py4j.examples.InterfaceB']

gateway = JavaGateway(start_callback_server=True)
gateway.entry_point.test(B())

And here's the Java code:
--------------------------------------------

public interface InterfaceA {
}

public interface InterfaceB {
	public InterfaceA foo();
}

public class ClassA implements InterfaceA{
}

public class InterfaceExample {

	public void test(InterfaceB b) {
		InterfaceA a = b.foo();
	}

	public static void main(String[] args) {
		GatewayServer server = new GatewayServer(new InterfaceExample());
		server.start();
	}
}

-------------------------------------
The error occurs in the line 'return A()':

py4j.protocol.Py4JJavaError: An error occurred while calling t.test.
: py4j.Py4JException: An exception was raised by the Python Proxy.
	at py4j.Protocol.getReturnValue(Protocol.java:393)
	at py4j.reflection.PythonProxyHandler.invoke(PythonProxyHandler.java:100)
	at $Proxy0.foo(Unknown Source)
	at py4j.examples.InterfaceExample.test(InterfaceExample.java:8)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at py4j.reflection.MethodInvoker.invoke(MethodInvoker.java:119)
	at py4j.reflection.ReflectionEngine.invoke(ReflectionEngine.java:392)
	at py4j.Gateway.invoke(Gateway.java:255)
	at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:124)
	at py4j.commands.CallCommand.execute(CallCommand.java:81)
	at py4j.GatewayConnection.run(GatewayConnection.java:175)
	at java.lang.Thread.run(Thread.java:662)