Subprocess.Popen does not give output

Question

I have the following code:

from subprocess import Popen, PIPE
if os.path.isfile('../'):
    cmd = os.path.join(os.getcwd(), '../')
    proc = Popen(["python3", "-m", cmd], stdout=PIPE, stderr=PIPE)
    progRunning = True
    while progRunning:
        try:
            output, error = proc.communicate()
        except KeyboardInterrupt:
            print("Program Forcequit")
            progRunning = False
            proc.kill()
    print('Process finished')

Where ../ is the directory to a file. This program is supposed to run ../ and stop itself when it receives a KeyboardInterrupt. However, when I try to run this code, print statements do not work. Here is the code in the ../ to test:

test.py
from time import *
while True:
    sleep(1)
    print("Hello World")

I am expecting an output of Hello World! every 1 second. I instead get no output. How do I fix this?

`../` is not a valid command, and [`os.getcwd()` is unnecessary](https://stackoverflow.com/questions/45591428/what-exactly-is-current-working-directory/66860904) — tripleee, Oct 16 '22 at 07:52
Bro can you read the whole post please? '../' is a placeholder name for the name of the file I want to run — PythonDudeLmao, Oct 16 '22 at 07:55
What makes you think I didn't (and what makes you think I'm a "bro")? — tripleee, Oct 16 '22 at 07:56
`communicate` finishes when the other process is finished. The other process is never finished in this case, so this just keeps waiting. — zvone, Oct 16 '22 at 08:10

score 3 · Accepted Answer · answered Oct 16 '22 at 07:54

There are several things wrong with this script. Firstly, the condition os.path.isfile('../') is always false, because .. is a directory, not a file. It should be os.path.isfile('../test.py'). Secondly, you should remove the -m option in the python call, because this loads test.py as a module, but does not execute it, so the script is never started. Thirdly, communicate is blocked until your script has finished, and the output is buffered, so nothing happens until the buffer is full (which can take quite some time). And finally, the output is put in the variable output, so in order to write the output to screen you should also add a print(output) in your main script.

I am wondering if running the test.py as an external python script is really required for your needs. Most probably you can better import as a normal python module

The reason why I can't use an import statement is because I want the program to execute when someone inputs the filename. When I try 'print output' it repeatedly returns an empty bytestring. — PythonDudeLmao, Oct 16 '22 at 08:00
I you are in control of the contents of test.py, you could add a function `run()` which starts the script. Then you can replace your Popen with test.run(). — Eelco van Vliet, Oct 16 '22 at 13:50

score 1 · Answer 2 · answered Oct 16 '22 at 09:38

1

As Eelco pointed out, Communicate blocks until process completion, So if u want to read and print the output of the child process try

while True:
    lines = proc.stdout.readlines()
    for line in lines:
        print (line)

answered Oct 16 '22 at 09:38

Abhilash Kar

24
2

When I try this I get a ValueError: I/O operation on closed file – PythonDudeLmao Oct 16 '22 at 09:46
That means Your subprocess gets terminated before/while this code executes. – Abhilash Kar Oct 16 '22 at 11:40

Subprocess.Popen does not give output

2 Answers2