How to redirect output from python using the DLR Hosting API

seshadripv — Tue, 08 Jul 2008 22:58:00 GMT

This is the 3rd in the series of 'How To' posts on using the DLR Hosting API to host languages like IronPython, IronRuby, Toyscript etc in a managed application. The first 2 posts are here and here.

The DLR Hosting API provides the ScriptIO class to give you complete control over the input, output and error streams used by the scripting languages. The Hosting API describes ScriptIO like this - “This class let's you control input and output by default for dynamic code running via DLR hosting. You can access the instance of this class from the IO property on ScriptRuntime.”

Here's the step by step guide to redirect the output to a Stream object using the ScriptIO class.

1) Create the runtime, the python engine and a MemoryStream object

ScriptRuntime runTime = ScriptRuntime.Create();

ScriptEngine pyEngine = runTime.GetEngine("py");

MemoryStream ms = new MemoryStream();

2) Set the memory stream created above as the output stream for this script runtime.( The API can redirect output to any Stream object. So you can also use a FileStream object to capture output).

runTime.IO.SetOutput(ms, new StreamWriter(ms));

3) Execute a python code snippet that would output something.

ScriptSource ss = pyEngine.CreateScriptSourceFromString(

"print 'hello world'",

SourceCodeKind.Statements);

ss.Execute();

4) Read from the memory stream object created in step (2).

string str = ReadFromStream(ms);

5) ‘str’ will now contain the output from the script.

The input and error streams can also be set using the above technique and calling the appropriate method in ScriptIO. Refer to the DLR Hosting API spec to learn more about the ScriptIO and other related classes.

The full sample follows. Normally you wouldn’t care to redirect the output, input or error streams in a console application. In spite of this, I am sticking to a console application as it lets me demonstrate the API usage succinctly and provide a full copy & paste enabled sample

using System;

using System.IO;

using System.Scripting;

using System.Text;

using Microsoft.Scripting.Hosting;

namespace ConsoleApplication1 {

class Program {

public static void Main(string[] args) {

ScriptRuntime runTime = ScriptRuntime.Create();

ScriptEngine pyEngine = runTime.GetEngine("py");

MemoryStream ms = new MemoryStream();

//runTime.IO refers to the current runtime's IO property

runTime.IO.SetOutput(ms, new StreamWriter(ms));

ScriptSource ss =

pyEngine.CreateScriptSourceFromString(

"print 'hello world'",

SourceCodeKind.Statements);

ss.Execute();

string str = ReadFromStream(ms);

ms.Close();

Console.WriteLine(str);

}

private static string ReadFromStream(MemoryStream ms) {

int length = (int)ms.Length;

Byte[] bytes = new Byte[length];

ms.Seek(0, SeekOrigin.Begin);

ms.Read(bytes, 0, (int)ms.Length);

return Encoding.GetEncoding("utf-8").GetString(bytes, 0, (int)ms.Length);

}

DLR Hosting and related stuff... : ScriptIO

How to redirect output from python using the DLR Hosting API