Is is possible to return an object or value from a Python script to the hosting application?

Question

For example in Lua you can place the following line at the end of a script:

return <some-value/object>

The value/object that is returned can then be retrieved by the hosting application.

I use this pattern so that scripts can represent factories for event handlers. The script-based event handlers are then used to extend the application. For example the hosting application runs a script called 'SomeEventHandler.lua' which defines and returns an object that is an event handler for 'SomeEvent' in your application.

Can this be done in Python? Or is there a better way to achieve this?

More specifically I am embedding IronPython in my C# application and am looking for a way to instance these script-based event handlers which will allow the application to be extended using Python.

Answer 1

It's totally possible and a common technique when embedding Python. This article shows the basics, as does this page. The core function is PyObject_CallObject() which calls code written in Python, from C.

Answer 2

This can be done in Python just the same way. You can require the plugin to provide a getHandler() function / method that returns the event handler:

class myPlugin(object):

  def doIt(self,event,*args):
    print "Doing important stuff"

  def getHandler(self,document):
    print "Initializing plugin"
    self._doc = document
    return doIt

When the user says "I want to use plugin X now," you know which function to call. If the plugin is not only to be called after a direct command, but also on certain events (like e.g. loading a graphics element), you can also provide the plugin author with possibilities to bind the handler to this very event.

Answer 3

The way I would do it (and the way I've seen it done) is have a function for each event all packed into one module (or across several, doesn't matter), and then call the function through C/C++/C# and use its return value.

Answer 4

See some examples in Embedding the Dynamic Language Runtime.

A simple example, setting-and-fetching-variables:

SourceCodeKind st = SourceCodeKind.Statements;
string source = "print 'Hello World'";
script = eng.CreateScriptSourceFromString(source, st);
scope = eng.CreateScope();
script.Execute(scope);
// The namespace holds the variables that the code creates in the process of executing it.
int value = 3;
scope.SetVariable("name", value);

script.Execute(scope);

int result = scope.GetVariable<int>("name");