Re: [Jython-users] jythonc can't return Python object

[Kevin B. <kb...@ca...>]

I should have clarified that slightly (I knew what I was saying, didn't you?)

The problem is the signature of the 'go' method.  It is declared to return 'ep', which jythonc can't find.  If it is declared to return 'Object', the compilation works just fine, but clients have to downcast 'Object' instances to 'ep' instances.

This isn't a problem for most jythonc users, because they are usually extending Java interfaces, rather than defining classes/interfaces in Python to be used in Java.

kb

Kevin Butler wrote:
> 
> dman's jythonc use brought up an interesting limitation in jythonc:
> 
> A java-object method cannot return an instance of a java-object subclass created by jythonc.
> 
> For example, in the following code, a method on 'ep' can't return an instance of 'ep' (the same failure occurs if the 'returned value' and the class-returning the value are separate classes in separate files.)
> 
> --- ep.py
> import java
> class ep( java.lang.Object ):
>     def go( self ):
>         """@sig public ep go()"""
>         return None
> 
> 
> jythonc complains that it cannot find the 'ep' class:
> 
> cd e:/work/kb/jythonc/extendPython/
> jythonc ep.py
> processing ep
> 
> Required packages:
>   java.lang
> 
> Creating adapters:
> 
> Creating .java files:
>   ep module
> Traceback (innermost last):
>   File "w:/tools/jython\Tools\jythonc\jythonc.py", line 5, in ?
>   File "w:\tools\jython\Tools\jythonc\main.py", line 300, in main
>   File "w:\tools\jython\Tools\jythonc\main.py", line 221, in doCompile
>   File "w:\tools\jython\Tools\jythonc\compile.py", line 381, in dump
>   File "w:\tools\jython\Tools\jythonc\compile.py", line 318, in processModule
>   File "w:\tools\jython\Tools\jythonc\compile.py", line 147, in makeJavaProxy
>   File "w:\tools\jython\Tools\jythonc\compile.py", line 53, in getsig
>   File "w:\tools\jython\Tools\jythonc\compile.py", line 84, in insistJavaClass
> ValueError: can not find class: ep
> 
> The reason this fails is that compile.py insists that any Java classes exist (presumably to prevent you from trying to return a Python object that isn't a java subclass - the generated Java code would fail in that case.) Because the generated code hasn't been compiled w/ javac yet, compile.py can't find the Java class, and throws the ValueError.
> 
> Is this a bug, a quirk, or "a bug but we're not about to fix it"?
> 
> :-)
> 
> kb
> 
> 
> 
> dman wrote:
> >
> > On Fri, Nov 02, 2001 at 03:50:12PM +0000, Finn Bock wrote:
> > | [dman]
> > |
> > | >I'm trying to use Jython(c) to allow using Moshe Zadka's PMS framework
> > | >along with a Java/Swing GUI for a school project.  (The other group
> > | >members are writing the gui and they don't know python)  I've run into
> > | >a couple of problems, though, all related to java's type system.
> > | >
> > | >There are several modules in the PMS package.  I have taken the base
> > | >classes of the inheritance tree and made them inherit from
> > | >java.lang.Object and added the @sig lines.  A simplified example :
> > | >
> > | >----- PMS/AbstractFolder.py
> > | >class AbstractFolder( java.lang.Object ) : pass
> > | >
> > | >----- PMS/Folder.py
> > | >import AbstractFolder
> > | >class Folder( AbstractFolder.AbstractFolder ) : pass
> > | >
> > | >
> > | >I also have the following class as a factory for the GUI to create a
> > | >hardcoded folder for the prototype demo.
> > | >
> > | >----- DemoFolder1.py
> > | >class DemoFolder1( java.lang.Object ) :
> > | >    def make( self , name ) :
> > | >        """
> > | >        @sig public Object make( String name )
> > | >        """
> > | >
> > | >        server = PMS.MaildirServer.MaildirServer( "server_name" , "." )
> > | >        PMS.Configuration.Configuration().servers[ server ] = lambda x : x
> > | >        folder = PMS.Folder.Folder( server , name )
> > | >        folder.create()
> > | >
> > | >        print folder.__class__ # correctly prints "PMS.Folder.Folder"
> > | >        return folder
> > | >
> > | >
> > | >
> > | >I used jythonc to compile the stuff, and everything compiles fine.
> > | >The Java code that uses this looks like :
> > | >
> > | >
> > | >            Object folder = (new DemoFolder1()).make( "Demo1" );
> > | >            MailMessage.Message message = new MailMessage.Message();
> > | >
> > | >            // This line prints out
> > | >            // org.python.proxies.PMS.Folder$Folder$2
> > | >            System.out.println( folder.getClass() ) ;
> > |
> > | The strange classname comes from the dynamic proxy generation.
> >
> > Yeah, the name is nicely self-documenting :-).
> >
> > | Verify if jythonc created a java class for the Folder class. You
> > | check that in the stdout from jythonc where there should be a line
> > | like:
> > |
> > |  Folder extends java.lang.Object
> > |
> > | If no such line is written, it is because jythonc failed to detect
> > | that Folder is a java subclass.
> >
> > What I see is :
> >
> >   AbstractFolder module
> >       AbstractFolder extends java.lang.Object
> >       NoSuchMessageError extends java.lang.Exception
> >   Folder module
> >       Folder extends AbstractFolder
> >
> > So Folder is indirectly a java subclass.
> >
> > I had originally tried
> >
> >     class Folder( java.lang.Object , AbstractFolder.AbstractFolder ) : pass
> >
> > but jythonc raises :
> >
> >     TypeError: no multiple inheritance for Java classes: org.python.proxies.AbstractFolder$AbstractFolder$6 and java.lang.Object
> >
> > Kevin Butler wrote to me off-list and mentioned trying the __tojava__
> > stuff.  This fails because I can't cast the above class to PyObject.
> > If I change the @sig to return PyObject, then the __tojava__ call
> > succeeds, but gives me a PySingleton instance, not a Folder instance.
> >
> > Is there a solution other than creating a python class to provide
> > pseudo-reflection to java?  (it takes a java.lang.Object and a name
> > and invokes it in python ; this works but isn't natural from the java
> > side)
> >
> > Thanks for your response!
> >
> > -D
> >
> > _______________________________________________
> > Jython-users mailing list
> > Jyt...@li...
> > https://lists.sourceforge.net/lists/listinfo/jython-users
> 
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