jython-users

[Jython-users] __findattr__ problem again

[Jakob S. <pj...@ip...>]

Hi again,

sorry for these repeated questions about __findattr__, but I just can't get
it to work properly. When I define __findattr__(String s) in my Java class,
I will get attribute errors for method calls from Jython on all methods in
my Java class. In the following example, which consists of two small files,
I will get

	AttributeError: instance of 'Test' has no attribute 'info'

Is this how __findattr__ is supposed, to work? Shouldn't the interpreter
first check whether the attribute exists as a method name before calling
__findattr__? (and if not, how do I combine __findattr__ with access to
member functions?)


Thanks,
Jakob



--- Main.java ---

import org.python.core.*;
import org.python.util.*;


public class Main {
  public static void main(String[] args) {
    PythonInterpreter p = new PythonInterpreter();
    PyObject ret;

      p.exec("import Test");
      p.exec("a = Test()");
      ret = p.eval("a.info()");

      System.out.println(ret.toString());
  }
}



--- Test.java ---

import org.python.core.*;

public class Test extends PyObject {

  public PyObject __findattr__(String s) {
    return null;
  }

  public String info() {
    return "info";
  }
}

Re: [Jython-users] __findattr__ problem again

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

[Jakob Svensson]

>sorry for these repeated questions about __findattr__, but I just can't get
>it to work properly. When I define __findattr__(String s) in my Java class,
>I will get attribute errors for method calls from Jython on all methods in
>my Java class. In the following example, which consists of two small files,
>I will get
>
>	AttributeError: instance of 'Test' has no attribute 'info'
>
>Is this how __findattr__ is supposed, to work?

Yes.

>Shouldn't the interpreter
>first check whether the attribute exists as a method name before calling
>__findattr__?

No.

>(and if not, how do I combine __findattr__ with access to
>member functions?)

The code "a.info()" get compiled into the call:

   a.invoke("info")

Where "invoke" is defined in PyObject as:

    public PyObject invoke(String name) {
        PyObject f = __getattr__(name);
        return f.__call__();
    }

and __getattr__ is defined as:

    public final PyObject __getattr__(PyString name) {
        PyObject ret = __findattr__(name);
        if (ret == null)
            throw Py.AttributeError(safeRepr() + " has no attribute '" +
                                    name + "'");
        return ret;
    }

So it is not at all surprising that you get an AttributeError. The
default implementation of __findattr__ in PyObject already support
lookup in the class and binding of functions.

A common implementation of __findattr__ in PyObject subclasses looks
something like this:

    public PyObject __findattr__(String name) {
        if (name == "foobar")
            return Py.newString("foobar");
        return super.__findattr__(name);
    }

First intercept the names that your object can answer for and then pass
the request to the class to resolve methods.

regards,
finn