Re: [Py4j-users] Interface troubles

[Barthelemy D. <ba...@cs...>]

Hi Alec,

Thanks for reporting this bug and for the test case, it was very helpful!

I just pushed a fix to the git repository and I added your test case in 
the unit test suite.

Don't hesitate to post again if you find another bug!

Barthélémy

On 03/16/2011 02:20 PM, Alec Chapman wrote:
> 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)
>
>
>
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