[SOLVED] tkinter - Can't assign window icon to root window if a prompt was created first

tkinter - Can't assign window icon to root window if a prompt was created first

Environment: Windows 10, Python 3.9.5

If I try to display a prompt (using tkinter.messagebox functions) before creating the root window, I cannot assign a window icon to the root window. The iconphoto call fails with the error, even though the icon given is a valid PhotoImage object:

_tkinter.TclError: can't use "pyimage1" as iconphoto: not a photo image

In this instance, my objective is to display a prompt to the user before the program starts, or even potentially ask a question (with tkinter.messagebox.askyesno) that may affect the operation of the GUI.

For example, the following code will cause this error:

## this is a base64 GIF to use as a window icon
icon_window_b64 = 'R0lGODlhIAAgAKECAP8AAP///wAAAAAAACH5BAEKAAIALAAAAAAgACAAAAJrlI+gy4oPX5sgWkFzu0f7en0iCI1faY5JKq5TEGRw5jLw3NzxZMj6rvgBR8LijQUwCpFBJY6ZVEIXzmnTaI1ip84nq3v8gsOmJfVHw7zIZ++iZhvG073spmNnoLKWPGfKUccUKHFCGNFyUQAAOw=='
## import tkinter
import tkinter
## import the messagebox module so we can make the prompt
import tkinter.messagebox
## show the prompt
tkinter.messagebox.showinfo('A prompt', 'Dismiss this for an exception')
## create the root window
root = tkinter.Tk()
## set the size of the root window
root.geometry('200x20')
## create the photoimage object from the base64 GIF
icon_window = tkinter.PhotoImage(data = icon_window_b64)
## put a label in to the window
tkinter.Label(root, text='This is a test').pack()
## set the window icon for the root window
root.iconphoto(False, icon_window)
## let the GUI block forevermore
root.mainloop()

However, this code will not:

## this is a base64 GIF to use as a window icon
icon_window_b64 = 'R0lGODlhIAAgAKECAP8AAP///wAAAAAAACH5BAEKAAIALAAAAAAgACAAAAJrlI+gy4oPX5sgWkFzu0f7en0iCI1faY5JKq5TEGRw5jLw3NzxZMj6rvgBR8LijQUwCpFBJY6ZVEIXzmnTaI1ip84nq3v8gsOmJfVHw7zIZ++iZhvG073spmNnoLKWPGfKUccUKHFCGNFyUQAAOw=='
## import tkinter
import tkinter
## import the messagebox module so we can make the prompt
import tkinter.messagebox
## create the root window
root = tkinter.Tk()
## show the prompt
tkinter.messagebox.showinfo('A prompt', 'No exceptions here!')
## set the size of the root window
root.geometry('200x20')
## create the photoimage object from the base64 GIF
icon_window = tkinter.PhotoImage(data = icon_window_b64)
## put a label in to the window
tkinter.Label(root, text='This is a test').pack()
## set the window icon for the root window
root.iconphoto(False, icon_window)
## let the GUI block forevermore
root.mainloop()

The key difference in the second code block is that the prompt is created after the tkinter.Tk() object. Obviously, I have few workarounds I can use:

Not use a window icon.
withdraw() the root immediately after creation so that it is not seen, and show it again after the call to the prompt so that it appears that the prompt came first.
Don't use the prompt functions at all (tkinter.messagebox, tkinter.dialog, etc), and question the user from elements inside the root (tkinter.Tk) object.

But, after all this, my question is: Is this expected behaviour, or is this a bug within tkinter? If it is expected behaviour, why is it?

Solution

It was a bug in the tkinter, fixed in Python 3.10.0. See the issue and the pull request for details.

In summary, with that fix, a temporary Tk root instance will be properly cleared, causing no side effects after showing and closing a dialog. (It's fine to have multiple Tk root instances created if they are used one at a time.)