Tk Source Code

There appears to be a problem with Tk not updating the change count of the system pasteboard correctly. This was discovered while attempting to copy data to the clipboard on MacOS from Python/Tkinter and paste the contents to BBedit.

Subsequent analasyis by the support team there highlighted where they believe the issue arises, and appears to trace to the Tk usage/implementation of the MacOS pastebin. I beleive that it therfore must lay somewhere within this file https://github.com/tcltk/tk/blob/main/macosx/tkMacOSXClipboard.c but lack the skill/knowledge to see exactly how.

To quote from their response

It appears that whatever mechanics are invoked by Tkinter when root.clipboard_clear() or root.clipboard_append are called are not updating the change count of the system pasteboard.

It's a best practice for pasteboard clients to check the change count (to test for external changes to the pasteboard) in advance of importing the system pasteboard; particularly with large pasteboard contents, this saves a great deal of time and memory.

BBEdit (and presumably many other applications which we haven't discovered yet) does this, and it observes that the pasteboard change count is the same as before; so it does not reimport the system pasteboard.

Full discussion can be found here. https://github.com/python/cpython/issues/104613

To recreate the issue this minimal python/Tkinter script can be used to load the clipboard with a new timestamp on each button press. Pasting the result into BBedit results in only the initial timestamp being pasted as it never re-imports the system pasteboard on subsequent copy/paste operations.

import tkinter as tk
import datetime

def on_click():
    root.clipboard_clear()
    now = datetime.datetime.now()
    print(now)
    root.clipboard_append(now)

root = tk.Tk()
frame = tk.Frame(root, padx=10, pady=10)
frame.pack()
btn = tk.Button(frame, text="Copy time", command=on_click)
btn.pack()
root.mainloop()

I've marked the severity as minor as so far I have only managed to observe this behaviour as affecting BBedit, but as their support team notes it could well affect many more undiscovered applications.

Thanks.