Accessing public attributes of Python class in Java with Jython

Question

to access Python objects in Java using Jython, it's necessary to create a Java interface and use coersion (PyObject.__java__()) to assign the object to a Java reference. (http://www.jython.org/faq2.html)

The problem is that the Python library I'm trying to use does not have gets and sets, and I don't want to change the library code if I can.

Is there a way to get those public variables without having to implement Python classes to subclass and expose those attributes through methods?

Thanks.

Answer 1

You can access the attributes from a PyObject directly by using the __getattr__(PyString) and __setattr__(PyString, PyObject) methods.

You can get an attribute with:

PyObject pyObject = ...;
// Get attribute: value = getattr(obj, name)
//            OR: value = obj.__getattr__(name)
PyString attrName = Py.newString("some_attribute");
PyObject attrValue = pyObject.__getattr__(attrName);

• WARNING: Make sure to use __getattr__(PyString) because __getattr__(String) only works for interned strings.

You can also set an attribute with:

PyObject pyObject = ...;
// Set attribute: setattr(obj, name, value)
//            OR: obj.__setattr__(name, value)
PyString attrName = Py.newString("some_attribute");
PyObject attrValue = (PyObject)Py.newString("A string as the new value.");
pyObject.__setattr__(attrName, attrValue);

• NOTE: The value does not have to be a PyString. It just has to be a PyObject.

• WARNING: Make sure to use __setattr__(PyString, PyObject) because __setattr__(String, PyObject) only works for interned strings.

Also, you can call a python method using __call__(PyObject[] args, String[] keywords):

PyObject pyObject = ...;

// Get method: method = getattr(obj, name)
//         OR: method = obj.__getattr__(name)
PyString methodName = Py.newString("some_method");
PyObject pyMethod = pyObject.__getattr__(methodName);

// Prepare arguments.
// NOTE: args contains positional arguments followed by keyword argument values.
PyObject[] args = new PyObject[] {arg1, arg2, ..., kwarg1, kwarg2, ...};
String[] keywords = new String[] {kwname1, kwname2, ...};

// Call method: result = method(arg1, arg2, ..., kwname1=kwarg1, kwname2=kwarg2, ...)
PyObject pyResult = pyMethod.__call__(args, keywords);

• NOTE: I cannot explain why the keyword names are String here when getting an attribute by name requires PyString.