Unable use tk.mainloop() in tkinter on ubuntu 16.04.2

Question

Whenever I try to run a simple tkinter program - or any program for that mater - using tk.mainloop() on ubuntu 16.04.2, I cannot open the app window and a question mark is shown over the window icon.

Here the the program I am running:

import tkinter as tk

class MainApplication(tk.Frame):
    def __init__(self, parent, *args, **kwargs):
        super(MainApplication, self).__init__(parent, *args, **kwargs)

if __name__ == '__main__':
    root = tk.Tk()
    MainApplication(root).pack(side="top", fill="both", expand=True)
    root.mainloop()

And here is a screen shot showing the output of the program whenever I run the it:

Before this, I was receiving an error saying _tkinter could be loaded. To remedy this, I downloaded the latest version of tkinter for ubuntu by using sudo apt-get update and then sudo apt-get install python3-tk. If this also matters, I'm using ubuntu in a dual-boot configuration with windows 10.

I've also read How can I fix program icons that appear as a question mark in the Launcher? and Why do some open applications appear as “question marks” in the Unity launcher?, but those question seemed to be dealing with how to fix a applications desktop icon, not how to fix an application that wouldn't run. Further more, the question seemed to be dealing with specific ubuntu applications.

Does anyone know why ubuntu is not correctly running the tkinter app and what can be done to fix this?

Answer 1

While it my not seem like it, The tkinter window is in fact there. It is simple to small to see because no widgets are inside of it to expand it. Once you began to add widgets to the window, it becomes visible.

For example, to make a window appear using the code in question,

import tkinter as tk


class MainApplication(tk.Frame):
    def __init__(self, parent, *args, **kwargs):
        super(MainApplication, self).__init__(parent, *args, **kwargs)


if __name__ == '__main__':
    root = tk.Tk()
    MainApplication(root).pack(side="top", fill="both", expand=True)
    root.mainloop() 

You could add a tk.Button() widget to force the window to expand:

import tkinter as tk


class MainApplication(tk.Frame):
    def __init__(self, parent, *args, **kwargs):
        super(MainApplication, self).__init__(parent, *args, **kwargs)
        # create a button widget to force the window to expand to fit
        # the button
        self.btn = tk.Button(self, text='btn', bg='blue')
        self.btn.pack(side='bottom')


if __name__ == '__main__':
    root = tk.Tk()
    MainApplication(root).pack(side="top", fill="both", expand=True)
    root.mainloop()

Which shows the following window:

Answer 2

Try to change the main portion of your program a little bit like this:

if __name__ == '__main__':
    root = tk.Tk()
    your_class_instance = MainApplication(root)
    print(your_class_instance)
    root.mainloop()

You will see on your Terminal something like this: .140449273960656

Now let us go back to your original code:

 if __name__ == '__main__':
        root = tk.Tk()
        your_class_instance = MainApplication(root).pack(side="top", fill="both", expand=True)
        print(your_class_instance)
        root.mainloop()

You will see None on your Terminal.

Why? That is because the layout managers of tkinter (pack(), place() and grid()) always return None and MainApplication class has no widgets.

In other words, you try to display nothing.

Note that pack(your_options_list) is useless in this line:

MainApplication(root).pack(side="top", fill="both", expand=True)

Not only it is useless but it prevents the application from further scalability because there is no way to use an instance of MainApplication class.

Maybe this is not necessary for your actual problem, but just as a side note, since you are using Python 3, you can use: super().__init__(parent, *args, **kwargs) instead of super(MainApplication, self).__init__(parent, *args, **kwargs)

Proving the provided explanation.

The explanation I provided for your problem (the above why?) states there are 2 facts that lead you to that situation. If one of them is satisfied, the problem disappears:

1. Leaving pack() in place but ask MainApplication to draw at least one widget:

   import tkinter as tk
   
   
   class MainApplication(tk.Frame):
       def __init__(self, parent, *args, **kwargs):
           super(MainApplication, self).__init__(parent, *args, **kwargs)
           # I add these 2 following lines
           self.parent = parent
           self.initialize_gui()
   
   def initialize_gui(self):
      # Ok, let us draw one useless button for a test
      self.button_1 = tk.Button(self.parent, text="Do nothing")
      self.button_1.pack()
   
   
   if __name__ == '__main__':
       root = tk.Tk()
       MainApplication(root).pack(side="top", fill="both", expand=True)
       root.mainloop()
   

You will see the GUI:

2. Let MainApplication class not drawing any widget but remove the pack():

   import tkinter as tk
   
   
   class MainApplication(tk.Frame):
       def __init__(self, parent, *args, **kwargs):
           super(MainApplication, self).__init__(parent, *args, **kwargs)       
   
   
   if __name__ == '__main__':
       root = tk.Tk()
       MainApplication(root) # Remove pack(...)
       root.mainloop()
   

You can see the GUI: