[Jython-users] How to use PythonInterpreter.getLocals() and setLocals()

[Byrd, W. <Wil...@co...>]

Hello all.

I am trying to embed jython in a Java app, but I have encountered a
stumbling block.

I want to instantiate and initialize a PythonInterpreter object, immediately
save the initialized state of the interpreter object, and periodically
restore that initialized state.  The PythonInterpreter getLocals() and
setLocals() methods seem to be just what I need:

		myInterpreter = new PythonInterpreter();
		
		// Set up the initial state of myInterpreter.
		myInterpreter.exec("from java.lang import Integer");
		... much more setup code here ...

		// Save the initial state of myInterpreter.
		PyObject initialState = myInterpreter.getLocals();

		// Change the state of myInterpreter.
		myInterpreter.exec("from java.util import Random");
		... much more code here ...

		// Try to restore the initial state of myInterpreter.
		// This doesn't work, since 'initialState' was modified 
		// by the previous invocation of exec().
		myInterpreter.setLocals(initialState);

Unfortunately, this code does not work.  The getLocals() method returns a
*reference* to the PythonInterpreter.locals object, rather than a deep copy.
The result is that the object referenced by 'initialState' is modified when
I make additional calls to PythonInterpreter.exec().

Peeking at the source code for PythonInterpreter allowed me to create the
following hack:

		... Set up the initial state of myInterpreter, as before ...

		// Save the initial state of myInterpreter.
		PyStringMap localsMap = (PyStringMap)
myInterpreter.getLocals();
		PyObject initialState = (PyObject) localsMap.copy();

		... Change the state of myInterpreter, as before ...

		// Restore the initial state of myInterpreter.  
		myInterpreter.setLocals(((PyStringMap)
initialState).copy());

Of course, this code will break if the return type of getLocals() ever
changes.

How can I achieve the same result without resorting to a hack?  Should
getLocals() return a deep copy of PythonInterpreter.locals?  Am I using the
getLocals() and setLocals() inappropriately?

Thanks.

Regards,

-- William Byrd