Why Do Tkinter Windows Appear Without Calling `mainloop()` in Interactive Shells?

Understanding mainloop in Tkinter

Every Tkinter application requires a call to mainloop to display windows and handle events. However, certain scenarios seem to defy this rule, as windows are drawn even without calling mainloop in interactive shells.

EventLoop

mainloop essentially establishes an infinite event loop. This loop continuously waits for events (e.g., mouse clicks, key presses) and processes them. It also checks for redraw requests and updates the widget's display.

Interactive Shell Convenience

When you run Tkinter code interactively, the shell itself takes care of running the event loop for you. This allows you to interact with the displayed window without the need to call mainloop explicitly. However, it's worth noting that all subsequent commands will block until the event loop ends.

Outside the Interactive Shell

If you run your code outside the interactive shell, you must call mainloop to start the event loop. Without it, the program will terminate before any events can be processed or updates displayed. This is especially critical when running scripts.

Conclusion

To summarize, calling mainloop is essential for any standalone Tkinter application to start the event loop and handle user interactions. However, the convenience of interactive shells abstracts away this requirement, allowing you to quickly test and debug code without needing to call mainloop explicitly.

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