Interact with a Windows console application via Python

Question

I am using python 2.5 on Windows. I wish to interact with a console process via Popen. I currently have this small snippet of code:

p = Popen( ["console_app.exe"], stdin=PIPE, stdout=PIPE )
# issue command 1...
p.stdin.write( 'command1\n' )
result1 = p.stdout.read() # <---- we never return here
# issue command 2...
p.stdin.write( 'command2\n' )
result2 = p.stdout.read()

I can write to stdin but can not read from stdout. Have I missed a step? I don't want to use p.communicate( "command" )[0] as it terminates the process and I need to interact with the process dynamically over time.

Thanks in advance.

Answer 1

Your problem here is that you are trying to control an interactive application.

stdout.read() will continue reading until it has reached the end of the stream, file or pipe. Unfortunately, in case of an interactive program, the pipe is only closed then whe program exits; which is never, if the command you sent it was anything other than "quit".

You will have to revert to reading the output of the subprocess line-by-line using stdout.readline(), and you'd better have a way to tell when the program is ready to accept a command, and when the command you issued to the program is finished and you can supply a new one. In case of a program like cmd.exe, even readline() won't suffice as the line that indicates a new command can be sent is not terminated by a newline, so will have to analyze the output byte-by-byte. Here's a sample script that runs cmd.exe, looks for the prompt, then issues a dir and then an exit:

from subprocess import *
import re

class InteractiveCommand:
    def __init__(self, process, prompt):
        self.process = process
        self.prompt  = prompt
        self.output  = ""
        self.wait_for_prompt()

    def wait_for_prompt(self):
        while not self.prompt.search(self.output):
            c = self.process.stdout.read(1)
            if c == "":
                break
            self.output += c

        # Now we're at a prompt; clear the output buffer and return its contents
        tmp = self.output
        self.output = ""
        return tmp

    def command(self, command):
        self.process.stdin.write(command + "\n")
        return self.wait_for_prompt()

p      = Popen( ["cmd.exe"], stdin=PIPE, stdout=PIPE )
prompt = re.compile(r"^C:\\.*>", re.M)
cmd    = InteractiveCommand(p, prompt)

listing = cmd.command("dir")
cmd.command("exit")

print listing

If the timing isn't important, and interactivity for a user isn't required, it can be a lot simpler just to batch up the calls:

from subprocess import *

p = Popen( ["cmd.exe"], stdin=PIPE, stdout=PIPE )
p.stdin.write("dir\n")
p.stdin.write("exit\n")

print p.stdout.read()

Answer 2

Have you tried to force windows end lines? i.e.

p.stdin.write( 'command1 \r\n' )
p.stdout.readline()

UPDATE:

I've just checked the solution on windows cmd.exe and it works with readline(). But it has one problem Popen's stdout.readline blocks. So if the app will ever return something without endline your app will stuck forever.

But there is a work around for that check out: http://code.activestate.com/recipes/440554/