changing the wallpaper in python program works when the program is ran directly. but doesn't work when it's run from a service

Question

I've been wanting to make a program that immediatly sets my wallpaper to 1 picture, waits 10 seconds, then sets it to the other.

The program works just fine when running it from command prompt, or using the python interpreter. But as a service, it doesn't work.

I have used nssm (Non-Sucking Service Manager) to turn the script into a service.

Here's the wallpaper changing part of the code:

def change_wallpaper(filename):
    # Load the user32 library.
    user32 = ctypes.WinDLL('user32')

    # Set the wallpaper.
    result = user32.SystemParametersInfoW(
        win32con.SPI_SETDESKWALLPAPER, 0, filename, win32con.SPIF_UPDATEINIFILE
    )

    # Check the result.
    if not result:
        print("Failed to set wallpaper.")
    else:
        print("Successfully set wallpaper.")

I captured the I/O to a log file to find out if it had worked or not and it said "Failed to set wallpaper".

So.. I'm kind of stuck. Thanks to anyone who can help. :)

---

What I was expecting

I was expecting it to change the wallpaper, then after 10 seconds change it to another.

What actually happened

It reported that changing the wallpaper failed.

Answer 1

Heads up, this is not my code. I've used a python wallpaper manager called superpaper before, and...it's a python wallpaper manager. The first rule of engineering is to not reinvent the wheel. Here is their project: https://github.com/hhannine/superpaper

import ctypes
from typing import List
import pythoncom
import pywintypes
import win32gui
from win32com.shell import shell, shellcon
user32 = ctypes.windll.user32


def _make_filter(class_name: str, title: str):
    """https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-enumwindows"""

    def enum_windows(handle: int, h_list: list):
        if not (class_name or title):
            h_list.append(handle)
        if class_name and class_name not in win32gui.GetClassName(handle):
            return True  # continue enumeration
        if title and title not in win32gui.GetWindowText(handle):
            return True  # continue enumeration
        h_list.append(handle)

    return enum_windows


def find_window_handles(parent: int = None, window_class: str = None, title: str = None) -> List[int]:
    cb = _make_filter(window_class, title)
    try:
        handle_list = []
        if parent:
            win32gui.EnumChildWindows(parent, cb, handle_list)
        else:
            win32gui.EnumWindows(cb, handle_list)
        return handle_list
    except pywintypes.error:
        return []


def force_refresh_syspar():
    user32.UpdatePerUserSystemParameters(1)


def enable_activedesktop():
    """https://stackoverflow.com/a/16351170"""
    try:
        progman = find_window_handles(window_class='Progman')[0]
        cryptic_params = (0x52c, 0, 0, 0, 500, None)
        user32.SendMessageTimeoutW(progman, *cryptic_params)
    except IndexError as e:
        raise WindowsError('Cannot enable Active Desktop') from e


def set_wallpaper_win(image_path: str, use_activedesktop: bool = True):
    if use_activedesktop:
        enable_activedesktop()
    pythoncom.CoInitialize()
    iad = pythoncom.CoCreateInstance(shell.CLSID_ActiveDesktop,
                                     None,
                                     pythoncom.CLSCTX_INPROC_SERVER,
                                     shell.IID_IActiveDesktop)
    iad.SetWallpaper(str(image_path), 0)
    iad.ApplyChanges(shellcon.AD_APPLY_ALL)
    force_refresh_syspar()

You would call it via

set_wallpaper_win('/path/to/image.png')

Answer 2

Another possible reason is that wallpapers are per user, and services don't necessarily run as the user. You want to set the service to run as the currently logged on user. From NSSM's website (as of 2022-12-28):

Log on tab The Log on tab can be used to manage the user account which will run the service. nssm will automatically ensure that the account you choose has the necessary log on as a service permissions.

Equivalent commands:

nssm set UT2003 ObjectName LocalSystem nssm set UT2003 Type SERVICE_WIN32_OWN_PROCESS Refer to the command line usage documentation for details on configuring an account and password on the command line. If you need to configure a blank password you must use the command line.

Additionally, it may be related to this question: Change wallpaper with service

Personally, I would just choose not to make it a Windows Service, and instead opt for a tray service or background CLI application. You can just make a shortcut in a startup folder that will call python with the arguments to start your python script.