Python Interactive Shell - outputting to stdout?

Steve Holden steve at
Mon Dec 27 16:21:25 CET 2004

Steve Holden wrote:

> Avi Berkovich wrote:
>> Hey,
>> I can't make it work, I don't get any data from either stdout nor stderr.
>> If I send lines and then close the stdin pipe, I may get an exception 
>> message from several lines up.
>> I tried manually reading from the stdout pipe, but it just blocks and 
>> hangs no matter what I send over via the stdin pipe.
>> This behavior isn't presented by the command line interpreter by any 
>> chance.
>> Any suggestions?
> Yes: post your code along with messages (if any)!
> regards
>  Steve

Sorry, for some reason I can no longer find the message you posted the 
code in -- it didn't get correctly threaded in Mozilla. Anyway, I can't 
say that I exactly understand what's going on, but here are a couple of 

1. The interpreter performs buffering even when running in interactive 
mode unless it can see a terminal (which here it can't). Hence adding a 
"-u" to the interpreter command line is useful.

2. There's a problem with your stop condition, which is why the attached 
modified version sleeps before closing the pipe to the interpreter.

import threading
import sys
import popen2

class Piper(threading.Thread):

    def __init__(self, readPipe, output):

        if not isinstance(readPipe, file):
            raise TypeError, "readPipe parameter must be of File type"
        #if not isinstance(output, file):
        #    raise TypeError, "output parameter must be of File type"

        self.readPipe = readPipe
        self.output = output
        self.toStop = False

    def run(self):
        print "Running"
        while not self.toStop and not self.readPipe.closed:
            read = self.readPipe.readline()
        print "Stopped"

    def stop(self):
        self.toStop = True

if __name__ == "__main__":
    r, w = popen2.popen4('c:\\python24\\python.exe -u')
    piper = Piper(r, sys.stdout)
    w.write("print 'Hello!'\r\n")
    w.write("print 'Goodbye!'\r\n")
    import time; time.sleep(2)
    #import time; time.sleep(3)

Various other bits and pieces of messing about didn't really yield any 
useful conclusions, so I pass this on only in the hope that you may be 
more motivated to follow up now you can actually see some interpreter 
output. I would have thought the flush() calls would have made output 
appear one line at a time, but sadly they do not.

Steve Holden     
Python Web Programming
Holden Web LLC      +1 703 861 4237  +1 800 494 3119

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