Question

Is it possible to declare IronPython classes as “Export” and thus add them to a MEF catalog that a
host C# application can Import?

I cannot really find any concrete examples of this, just speculation.

Here is how I have manually loaded a Python class that implements a .NET interface:

https://github.com/versionone/VersionOne.SDK.Experimental

I would like to be able to put attributes on the python classes similar to how you do it in C#.
(Or something equivalent)

Has anyone tried this?

Thanks,
Josh

Answer 1

For those interested, I found a project on GitHub that had done this, but was a little bit coupled into the project. With the author’s approval I’ve created a new repo, IronPythonMef, and a NuGet package for it.

There’s additional discussion in this thread at GitHub.

Here’s an example of how it works:

First, an interface declared in C#:

namespace IronPythonMef.Tests.Example.Operations
{
    public interface IOperation
    {
        object Execute(params object[] args);
        string Name { get; }
        string Usage { get; }
    }
}

An implementation that Exports that interface in C#:

[Export(typeof(IOperation))]
public class Power : IOperation
{
    public object Execute(params object[] args)
    {
        if (args.Length < 2)
        {
            throw new ArgumentException(Usage, "args");
        }

        var x = Convert.ToDouble(args[0]);
        var y = Convert.ToDouble(args[1]);

        return Math.Pow(x, y);
    }

    public string Name
    {
        get { return "pow"; }
    }

    public string Usage
    {
        get { return "pow n, y -- calculates n to the y power"; }
    }
}

And, an implementation of IOperation in IronPython:

@export(IOperation)
class Fibonacci(IOperation):
    def Execute(self, n):
        n = int(n)
        if n == 0:
            return 0
        elif n == 1:
            return 1
        else:
            return self.Execute(n-1) + self.Execute(n-2)

    @property
    def Name(self):
        return "fib"

    @property
    def Usage(self):
        return "fib n -- calculates the nth Fibonacci number"

And here’s a test case of a class that imports these operations from both C# and IronPython and executes them:

[TestFixture]
public class MathWizardTests
{
    [Test]
    public void runs_script_with_operations_from_both_csharp_and_python()
    {
        var mathWiz = new MathWizard();

        new CompositionHelper().ComposeWithTypesExportedFromPythonAndCSharp(
            mathWiz,
            "Operations.Python.py",
            typeof(IOperation));

        const string mathScript =
@"fib 6
fac 6
abs -99
pow 2 4
";
        var results = mathWiz.ExecuteScript(mathScript).ToList();

        Assert.AreEqual(8, results[0]);
        Assert.AreEqual(720, results[1]);
        Assert.AreEqual(99f, results[2]);
        Assert.AreEqual(16m, results[3]);
    }
}