Can't run second python file via subprocess

I need to run send_mess.py from my main application main.py so that the two applications run in parallel to each other.

def mailing_launch(self):
    global p
    p = subprocess.Popen(['python', 'send_mess.py'])
    self.cancel_button.configure(state=customtkinter.NORMAL)
    self.start_button.configure(state=customtkinter.DISABLED)

This code worked fine, but when I moved the project to another computer, it refuses to run:

FileNotFoundError: [Errno 2] No such file or directory: 'python'

If I use

p = subprocess.Popen(['send_mess.py'])

it I get an error:

FileNotFoundError: [Errno 2] No such file or directory: 'send_mess.py'

All files are in the same shared project folder.

My system does not have python, so I assume that python is not always available. The sure thing to use instead if sys.executable which points to where the Python interpreter is, so replace that one line you have with:

    p = subprocess.Popen([sys.executable, 'send_mess.py'])

You can get the path to the current interpreter with sys.executable. Also, if you are running the script from a different working directory, you might have to parse the path to your other script as well, I did this with pathlib, assuming both files are in the same folder

import subprocess
import sys
from pathlib import Path

python_path = sys.executable
script_path = Path(__file__).parent.joinpath("send_message.py")

p = subprocess.Popen([sys.executable, script_path])
print("do other stuff")
p.wait()