[Jython-dev] invoking __getattr__()/__setattr__() on java objects ?

[Oti <oh...@ya...>]

Hello developers,


Motivation:
-----------

if a java class X implements methods
  __add__()
  __sub__(), ...
these methods are invoked if i use
  x = X()
  x + 10
  x - 10
in the interpreter.

Similarly, if X implements methods
  __eq__(),
  __lt__(), ...
these methods are invoked if i use
  x = X()
  x == 10
  x < 10
in the interpreter.

This is cool, pretty cool i'd say !
In both cases it is the java class' responsibility to do and/or return
something useful.


Idea / Question:
----------------

Now it would be even cooler if methods
  __getattr__()
  __setattr__()
would be invoked if i use
  x = X()
  x.undefinedAttribute
  x.undefinedAttribute = 10
  x.undefinedMethod()
Again it would be the java class' responsibility to do and/or return
something useful. 
What do you think of this idea ?



It can be done:
---------------

The following two little changes (against the 2.1 codebase) enable the
above.
I am using this extensively for much closer Jython/Java integration.
Is it worth opening a patch ?



1) File /core/PyJavaInstance.java:

Old method:

  protected void noField(String name, PyObject value) {
    throw Py.TypeError("can't set arbitrary attribute in java instance:
"+
                       name);
  }

replaced with:

  protected void noField(String name, PyObject value) {
    PyObject method = __class__.lookup("__setattr__", false);
    if ( method == null ) {
      throw Py.TypeError("can't set arbitrary attribute in java
instance: " + name);
    } else {
      method.__call__(this, new PyString(name), value);
    }
  }


2) File /core/PyInstance.java:

Old method:

  protected PyObject ifindfunction(String name) {
    PyObject getter = __class__.__getattr__;
    if (getter == null)
      return null;
    
    try {
      return getter.__call__(this, new PyString(name));
    } catch (PyException exc) {
      if (Py.matchException(exc, Py.AttributeError)) return null;
      throw exc;
    }
  }

replaced with:

  protected PyObject ifindfunction(String name) {
    PyObject getter = __class__.__getattr__;
    if ( getter == null  &&  __class__.getProxyClass() != null ) {
      getter = __class__.lookup("__getattr__", false);
    }
    if ( getter == null ) {
      return null;
    }
    
    try {
      return getter.__call__(this, new PyString(name));
    } catch (PyException exc) {
      if (Py.matchException(exc, Py.AttributeError)) return null;
      throw exc;
    }
  }


Many thanks for your feedback.

Always-trying-to-make-jython-even-more-useful-ly yours
Oti.

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