Why are multiple instances of Tk discouraged?

I disagree with the tkinter community discouraging the use of multiple tk.Tk windows. You can have multiple tk.Tk windows. Using multiple instances of tk.Tk is the only way to create windows that are truly independent of each other. The most only mistake people do when creating multiple tk.Tk windows is that they forget to pass in master=... when creating PhotoImages/StringVars/IntVars/…

For example look at this code:

import tkinter as tk

root = tk.Tk()
root2 = tk.Tk()

variable = tk.StringVar() # Add `master=root2` to fix the problem
entry = tk.Entry(root2, textvariable=variable)
entry.bind("<Return>", lambda e: print(repr(variable.get())))


The code above doesn’t work. If you add master=root2 to the tk.StringVar(), then it will work perfectly fine. This is because tkinter stores the first instance of tk.Tk() in tk._default_root. Then if you don’t pass in master=..., it will assume that you wanted the window in tk._default_root.

Another thing people get wrong is how many times should .mainloop() be called. It handles events from all tk.Tk windows that are alive so you only need one .mainloop().

For folks who disagree, I’d be interested in an example of where an actual problem is caused by the multiple tk.Tk windows.

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