interacting with python from c# application

Question

I would like to write a C# application that interacts with a python process. See sample code below. After starting the python process and writing to its StdIn, nothing happens until the StdIn stream is closed. When closed, the code submitted via StdIn is executed then the python process closes. How can I get python to execute the code submitted via StdIn without having to first close the StdIn?

StringBuilder strOut =  new StringBuilder();
ProcessStartInfo processStartInfo =  new ProcessStartInfo();
Process process = new Process();

processStartInfo.CreateNoWindow = true;
processStartInfo.RedirectStandardOutput = true;
processStartInfo.RedirectStandardInput = true;
processStartInfo.RedirectStandardError = true;
processStartInfo.UseShellExecute = false;
processStartInfo.Arguments = "";
processStartInfo.FileName = "c:\\Python39\\python.exe";

process.StartInfo = processStartInfo;
process.EnableRaisingEvents = true;
process.OutputDataReceived += new DataReceivedEventHandler
(
    delegate (object sndr, DataReceivedEventArgs ee)
    {
        Console.WriteLine("received=" + ee.Data);
        strOut.Append(ee.Data);
    }
);

process.ErrorDataReceived += new DataReceivedEventHandler
(
    delegate (object sndr, DataReceivedEventArgs ee)
    {
        Console.WriteLine("error=" + ee.Data);
        strOut.Append("Error:" + ee.Data);
    }
);

process.Start();
StreamWriter wrtr = process.StandardInput;
process.BeginOutputReadLine();
process.BeginErrorReadLine();
wrtr.WriteLine("import sys");
wrtr.WriteLine("print('Line 1')");
Thread.Sleep(1000);
wrtr.WriteLine("prxint('Line 2')");
wrtr.WriteLine("sys.exit('Test Exit')");
wrtr.Close();
Thread.Sleep(1000);
process.WaitForExit();
process.CancelOutputRead();

string output = strOut.ToString();

You should also post minimal reproducible example of the Python's code that shows some basic interactive processing (like read string and immediately print it in an infinite loop). — Alexei Levenkov
– Alexei Levenkov, Commented Jul 27, 2021 at 19:45
The code provided in minimal reproducible. The Python code is embedded in the StdIn writer statements (see C# wrtr object). To put it another way, the question is how can I get the python code submitted via the StdIn writer (wrtr object) to execute immediately rather than wait until the process is closed by closing the wrtr object? — rka
– rka, Commented Jul 27, 2021 at 20:28
Most likely you missed something reading result of "python -?". Consider re-reading and investigate from there. — Alexei Levenkov
– Alexei Levenkov, Commented Jul 27, 2021 at 20:44

ZiggZagg · Accepted Answer · 2021-07-27 21:17:06Z

1

Python tries to detect what environment it is running under. It switches to non interactive mode because it is not run in a terminal window due to UseShellExecute = false. You can enforce interactive mode by passing the -i argument: processStartInfo.Arguments = "-i";

You can add -q as well to disable to the copyright and version message

answered Jul 27, 2021 at 21:17

ZiggZagg

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1 Comment

rka Over a year ago

The -i argument solved the problem. Thank you very much.

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