Question

I’m writing a simple wrapper over python debugger (pdb) and I need to parse pdb output. But I have a problem reading text from process pipe.

Example of my code:

import subprocess, threading, time

def readProcessOutput(process):
    while not process.poll():
        print(process.stdout.readline())

process = subprocess.Popen('python -m pdb script.py', shell=True, universal_newlines=True,
                           stdout=subprocess.PIPE, stderr=subprocess.STDOUT, stdin=subprocess.PIPE)
read_thread = threading.Thread(target=readProcessOutput, args=(process,))
read_thread.start()

while True:
    time.sleep(0.5)

When i execute given command (python -m pdb script.py) in OS shell I get results like this:

> c:\develop\script.py(1)<module>()
-> print('hello, world!')
(Pdb)

But when i run my script i get only two lines, but can’t get pdb prompt. Writing commands to stdin after this has no effect. So my question is:
why I cannot read third line? How can I avoid this problem and get correct output?

Platform: Windows XP, Python 3.3

Answer 1

The third line can not be read by readline() because it is not terminated yet by the end of line. You see usually the cursor after “(pdb) ” until you write anything + enter.

The communication to processes that have some prompt is usually more complicated. It proved to me to write also an independent thread for data writer first for easier testing the communication in order to be sure that the main thread never freezes if too much is tried to be written or read. Then it can be simplified again.