Environment:

Windows, Python 3.12

Problem Summary:

I have a Python application that utilizes a multiprocessing.Pool to process a bunch of files in parallel:

proc_count = 16
with multiprocessing.Pool(proc_count) as pool:
    result = pool.map_async(parse_func, logs)
    # ... processing of result.get()

When run from the command line, everything is fine (no extra windows).

But when the app is launched with no console (from our launcher, a NodeJS Electron app that spawns the child process with shell=False), a python.exe window pops up for each process (so 16 windows in this case).

I don’t want to set shell=True; The app has a GTK4 front-end, and I don’t want a console window in addition to the app’s GUI.

Is there any exposed way to disable window creation by the multiprocessing.Pool?

What Hasn’t Worked:

I have tried passing an initializer function to the Pool that attempts to hide the window, but it had no effect:

# Testing showed result = 0 below
# From Microsoft's docs: If the window was previously hidden, the return value is zero.
def initializer():
    window = win32gui.GetForegroundWindow()
    title = win32gui.GetWindowText(window)
    if title.endswith('python.exe'):
        result = win32gui.ShowWindow(window, win32con.SW_HIDE)

What Has Worked (but it’s bad):

I did find a way to make this work by setting the process creation flags (normally set to 0) in the multiprocessing library itself, but obviously I’m looking for a better solution.

From modified PythonDir/Lib/multiprocessing/popen_spawn_win32.py:

with open(wfd, 'wb', closefd=True) as to_child:
    # start process
    try:
        CREATE_NO_WINDOW = 0x08000000  # <-- my temporary hack
        hp, ht, pid, tid = _winapi.CreateProcess(
            python_exe, cmd,
            None, None, False, CREATE_NO_WINDOW, env, None, None)  # <-- dwCreationFlags was 0 before