Custom IronPython import resolution

Question

I am loading an IronPython script from a database and executing it. This works fine for simple scripts, but imports are a problem. How can I intercept these import calls and then load the appropriate scripts from the database?

EDIT: My main application is written in C# and I'd like to intercept the calls on the C# side without editing the Python scripts.

EDIT: From the research I've done, it looks like creating your own PlatformAdaptationLayer is the way you're supposed to to implement this, but it doesn't work in this case. I've created my own PAL and in my testing, my FileExsists method gets called for every import in the script. But for some reason it never calls any overload of the OpenInputFileStream method. Digging through the IronPython source, once FileExists returns true, it tries to locate the file itself on the path. So this looks like a dead end.

Answer 1

After a great deal of trial and error, I arrived at a solution. I never managed to get the PlatformAdaptationLayer approach to work correctly. It never called back to the PAL when attempting to load the modules.

So what I decided to do was replace the built-in import function by using the SetVariable method as shown below (Engine and Scope are protected members exposing the ScriptEngine and ScriptScope for the parent script):

delegate object ImportDelegate(CodeContext context, string moduleName, PythonDictionary globals, PythonDictionary locals, PythonTuple tuple);

protected void OverrideImport()
{
    ScriptScope scope = IronPython.Hosting.Python.GetBuiltinModule(Engine);
    scope.SetVariable("__import__", new ImportDelegate(DoDatabaseImport));
}

protected object DoDatabaseImport(CodeContext context, string moduleName, PythonDictionary globals, PythonDictionary locals, PythonTuple tuple)
{
    if (ScriptExistsInDb(moduleName))
    {
        string rawScript = GetScriptFromDb(moduleName);
        ScriptSource source = Engine.CreateScriptSourceFromString(rawScript);
        ScriptScope scope = Engine.CreateScope();
        Engine.Execute(rawScript, scope);
        Microsoft.Scripting.Runtime.Scope ret = Microsoft.Scripting.Hosting.Providers.HostingHelpers.GetScope(scope);
        Scope.SetVariable(moduleName, ret);
        return ret;
     }
     else
     {   // fall back on the built-in method
         return IronPython.Modules.Builtin.__import__(context, moduleName);
     }
}

Hope this helps someone!