Can't get an output to Tkinter GUI

Question

I have an issue with Tkinter, I'm running this script "index.py" after clicking the button, the script starts running, but I do not get an output anywhere, any remarks?

from tkinter import *
import threading
import subprocess

root = Tk()

frame = Frame(root, width=300, height=300)
frame.pack()


def myClick():

    t = threading.Thread(target=run())
    t.start()


def run():
    arg = "python index.py"
    process = subprocess.check_output(arg)

    lab = Label(frame, text=process)
    lab.pack()


myButton = Button(root, text="run", padx=40, pady=10, command=myClick)
myButton.pack(pady=40)

root.mainloop()

Answer 1

The way you are starting a subprocess does not work on all platforms.

As acw1668 commented, it works fine on ms-windows. But it does not work on POSIX platforms like macOS, Linux or *BSD:

> python
Python 3.9.13 (main, May 31 2022, 12:56:40) 
[Clang 13.0.0 (git@github.com:llvm/llvm-project.git llvmorg-13.0.0-0-gd7b669b3a on freebsd13
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> arg = "python index.py"
>>> process = subprocess.check_output(arg)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/subprocess.py", line 424, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/local/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/local/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/local/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'python index.py'
>>> 

This is due to the different way that process creation works on different platforms.

Look closely at the last line of the error message. You have told subprocess to execute a file named python<space>index.py, which of course doesn't exist.

Convert arg into a list of strings, and it will work:

>>> arg = ["python", "index.py"]
>>> process = subprocess.check_output(arg)
>>> print(process)
b'Hello from index.py\n'

The following code should work everywhere:

def run():
    arg = ["python", "index.py"]
    process = subprocess.check_output(arg)

    lab = Label(frame, text=process)
    lab.pack()

EDIT1:

IDE's can do weird things with the standard output of your programs. And they might interact with scripts using subprocess, multiprocessing or threading in non-inuitive ways. When in doubt, first run your program from a command prompt. If it works there, it is an IDE problem.

In general, if you have any problem with a python script in an IDE or interactive environment, try saving it to a file and running it from a command prompt first.

Answer 2

That's because at line 12, you use target=run() instead of target=run

Answer 3

I dont know what you are trying to achieve exactly, but the issue is with the process line.

So this correction works:

import threading
import subprocess

from tkinter import *

root = Tk()

frame = Frame(root, width=300, height=300)
frame.pack()


def myClick():

    t = threading.Thread(target=run())
    t.start()


def run():
    
    arg = "python index.py"
    # process = subprocess.check_output(arg)   # <--- error is here

    lab = Label(frame, text='hello')   # <---  this works
    lab.pack()




myButton = Button(root, text="run", padx=40, pady=10, command=myClick)
myButton.pack(pady=40)

root.mainloop()

So the result looks like this:

Every time you click it says "hello".