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terminal_spawner

Spawning a child process from a parent in Python (using the subprocess module) is not a walk in the park. There are lots of variables to take into account:

  • Are you running on Windows or Linux?
  • Is the parent process a live Python script (invoked directly with your Python interpreter from a console)? Or is it a frozen executable?
  • How about the child?
  • Should the child process "die" once the parent stops?
  • Should the child process launch its own console?
  • ...

image

To get a better grasp on the situation, I decided to create this terminal_spawner repository.

 

1. Repo overview

This repository contains two Python applications: the Parent App and Child App. Following Python files are involved:

  • parent_app.py: Main Python file from the Parent App.
  • child_app.py: Main Python file from the Child App.
  • build.py: Run this script to build both the parent and child applications with cx_freeze. The parent ends up in the folder frozen_parent_app/, the child in frozen_child_app/.
  • functions.py: A help-script containing Python functions used in both the parent and child apps.

image

 

2. Build the Parent and Child Apps

To build both the Parent App and Child App, simply invoke the build.py script:

$ python build.py [--no-console]

This should then create the folders frozen_parent_app/ and frozen_child_app/:

image

You can choose to add the --no-console parameter when invoking the build script. This parameter should only be used on Windows. It results in passing base = 'Win32GUI' to cx_freeze. We'll see further on that the addition of this parameter will have an important impact on the final behavior of the application!

 

3. Launch the Parent App

3.1 Launch as Live Python Script

To launch the Parent App, simply invoke it with Python:

$ python parent_app.py [--foo] [--bar "some text"]

You can add two arguments if you wish to do so:

  • --foo: An optional argument of boolean nature. Either it's present or absent.
  • --bar: An optional argument that expects a string value.

A simple PyQt6 application starts and prints the arguments you passed - among other things:

image

3.2 Launch as Executable

Alternatively, you can launch the Parent App as a (frozen) executable. Make sure you run the build.py script first (see previous chapter). Then navigate into the frozen_parent_app/ folder and launch the executable from there:

$ cd frozen_parent_app
$ parent_app.exe [--foo] [--bar "some text"]

Again, you can add the foo and/or bar arguments if you wish. The application starts. This time, the app recognizes that it's running from an executable and shows that by setting its Frozen value True:

image

Also, the app colors itself blue to point out that it's ... frozen!

 

4. Launch the Child App

You can launch the Child App in the exact same way as you did for the Parent App: either as a live Python script or by invoking its (frozen) executable after a build. But then you'd be missing the point. The whole idea of the terminal_spawner project is that you launch the child application from within the parent.

4.1 Launch as Live Python Script vs Executable

Suppose the Parent App is already running. You'll see two buttons at the bottom:

image

The first one looks for the child's Python script and runs it as such - invoking your default Python interpreter. The second button searches for the child's application executable in the frozen_child_app/ subfolder and launches it from there.

4.2 Checkboxes

With the checkboxes, you can modify the behavior:

image

  • Pass own foo and bar args to child: This option is straightforward. Check it to pass the arguments given to the parent also to the child when it spawns.

  • Run waitfunc() after spawning child: The subprocess.Popen() function returns a subprocess-object. Check this box if you want to invoke the wait() function from that subprocess-object right after creating it.

  • Quit after spawning child: Check this box if you want the parent application to quit immediately after spawning the child (or after spawning and running the wait() function - see previous box). This is a very interesting experiment. It's basically what we do in Embeetle when switching from the main app to the updater tool - and back. The experiment succeeds if the child app stays alive even though the parent app disappears.

 

5. The --no-console Parameter

Remember the way you could build both the parent and child apps:

$ python build.py [--no-console]

If you add --no-console here, the argument base = 'Win32GUI' is passed to cx_freeze. This has a profound effect on the behavior of the frozen app. The button PRINT INFO TO CONSOLE no longer works - which is pretty normal. However, it also causes the app to crash when you try to spawn a child!

NOTE: In Python 3.9, it wouldn't crash. In Python 3.12 it does.

 

6. How the Parent Spawns its Child

The code to spawn a child process is all in functions.py. As you will see, this goes further than simply invoking supbrocess.Popen(). There's a whole mechanism to not merely launch the child application, but launch it in a console. For Embeetle this is important. The child application doesn't necessarily have a GUI. For example - the Beetle Updater Tool is a terminal-only application.

The main function says it all:

def spawn_new_terminal(script_or_exe_path:str, argv:List[str], **kwargs) -> Callable:
    '''
    Spawn a new terminal and launch the given script (python or shell script) or executable in that
    terminal. This function returns a callable 'wait_function()' that the parent process (which
    invoked this function) can use to wait for the child process (which runs in the newly spawned
    terminal) to complete. That 'wait_function()' returns the 'returncode', which is 0 if all was
    good.

    :param script_or_exe_path:  The script (python or shell script) or executable to be launched in
                                the newly spawned terminal. For example:
                                  - 'C:/users/krist/child.py'  or '/home/krist/child.py'
                                  - 'C:/users/krist/child.bat' or '/home/krist/child.sh'
                                  - 'C:/users/krist/child.exe' or '/home/krist/child'

    :param argv:                The arguments to be passed to the script or executable. Do not
                                include the (path to the) script file or executable in here. Just
                                the arguments.
    '''
    [...]

The function spawn_new_terminal() needs two arguments:

  • script_or_exe_path: The path to the script or executable to be run.
  • argv: The arguments to be passed to the script or executable.

The function tries to be as generic as possible. You can pass it a Python script, an executable or even a shell script. It will figure out what it gets and act accordingly. Then it launches said script/exe as a child process in its own dedicated console. On Windows that would be the standard CMD console. On Linux it looks for what's available.