Question

I’ve written a simple program that captures and executes command line Python scripts, but there is a problem. The text passed to a Python input function isn’t written to my program despite my program capturing stdout.

For example:
The Python script:

import sys

print("Hello, World!")
x = input("Please enter a number: ")
print(x)

print("This work?")

Would write “Hello, World!” then stop. When I pass it a number it would continue on writing “Please enter a number: 3”. What is going on? Any solutions? My C# is as follows:

public partial class PyCon : Window
{
        public string strPythonPath;
        public string strFile;
        public string strArguments;
        private StreamWriter sw;

        public PyCon(string pythonpath, string file, string args)
        {
            strPythonPath = pythonpath;
            strFile = file;
            strArguments = args;

            InitializeComponent();

            Process p = new Process();

            p.StartInfo.FileName = strPythonPath;
            p.StartInfo.Arguments = "\"" + strFile + "\" " + strArguments;

            p.StartInfo.UseShellExecute = false;
            p.StartInfo.CreateNoWindow = true;

            p.StartInfo.RedirectStandardInput = true;
            p.StartInfo.RedirectStandardOutput = true;
            p.StartInfo.RedirectStandardError = true;

            p.OutputDataReceived += new DataReceivedEventHandler(p_OutputDataReceived);
            p.ErrorDataReceived += new DataReceivedEventHandler(p_ErrorDataReceived);

            p.Start();
            p.BeginOutputReadLine();
            p.BeginErrorReadLine();
            sw = p.StandardInput;
        }

        private void p_OutputDataReceived(object sendingProcess, DataReceivedEventArgs received) {
            if (!String.IsNullOrEmpty(received.Data)) {
                AppendConsole(received.Data);
            }
        }

        private void p_ErrorDataReceived(object sendingProcess, DataReceivedEventArgs received) {
            if (!String.IsNullOrEmpty(received.Data)) {
                AppendConsole(received.Data);
            }
        }

        private void AppendConsole(string message) {
            if (!txtConsole.Dispatcher.CheckAccess()) {
                txtConsole.Dispatcher.Invoke(DispatcherPriority.Normal, (System.Windows.Forms.MethodInvoker)delegate() { txtConsole.AppendText(message + "\n"); });
            } else {
                //Format text
                message = message.Replace("\n", Environment.NewLine);

                txtConsole.AppendText(message + "\n");   
            }
        }

        private void txtInput_KeyUp(object sender, KeyEventArgs e) {
            if (e.Key != Key.Enter) return;

            sw.WriteLine(txtInput.Text);

            txtInput.Text = "";


        }
    }

Edit: After a lot of research and help from this thread, I’ve come to the conclusion that the problem is with the Python input command not calling the C# DataReceivedEventHandler. There may not be a solution to this besides scripting changes. If that is the case, I’ll make the answer containing those changes as accepted. Thanks for the help, guys!

Answer 1

Smells like the Python i/o is line buffered, i.e. waits for a CRLF then sends a whole line at once. You could try turning that off (python -u myscript.py, or set the PYTHONUNBUFFERED environment variable) or work around it with something like this:

print("Hello, World!")
print("Please enter a number: ")
x = input()
print(x)