code-review-tk-switch-frames.md

Switching frames in tkinter by passing class objects

A question on Stack Overflow asked how to create a tkinter GUI in Python 3 that would switch between two frames: A start-page and a sub-page.

Objectives

• Create a Python 3 app which switches between multiple tkinter frames
• Code should be readable and easy to understand
• Scaleable to larger apps where classes are split across multiple files

My Approach

I answered with a sample app that switches between a main page and two sub-pages. The concept is, have a parent Tk class (master) with a switch_frame() method. When called with a frame class, this method will create a new frame to replace the old one.

Code

# Multi-frame tkinter application v2.3
import tkinter as tk

class SampleApp(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)
        self._frame = None
        self.switch_frame(StartPage)

    def switch_frame(self, frame_class):
        """Destroys current frame and replaces it with a new one."""
        new_frame = frame_class(self)
        if self._frame is not None:
            self._frame.destroy()
        self._frame = new_frame
        self._frame.pack()

class StartPage(tk.Frame):
    def __init__(self, master):
        tk.Frame.__init__(self, master)
        tk.Label(self, text="This is the start page").pack(side="top", fill="x", pady=10)
        tk.Button(self, text="Open page one",
                  command=lambda: master.switch_frame(PageOne)).pack()
        tk.Button(self, text="Open page two",
                  command=lambda: master.switch_frame(PageTwo)).pack()

class PageOne(tk.Frame):
    def __init__(self, master):
        tk.Frame.__init__(self, master)
        tk.Label(self, text="This is page one").pack(side="top", fill="x", pady=10)
        tk.Button(self, text="Return to start page",
                  command=lambda: master.switch_frame(StartPage)).pack()

class PageTwo(tk.Frame):
    def __init__(self, master):
        tk.Frame.__init__(self, master)
        tk.Label(self, text="This is page two").pack(side="top", fill="x", pady=10)
        tk.Button(self, text="Return to start page",
                  command=lambda: master.switch_frame(StartPage)).pack()

if __name__ == "__main__":
    app = SampleApp()
    app.mainloop()

While this small example works, I'm concerned this approach could result in circular references if scaled to a larger application with multiple files. (eg: StartPage imports PageOne, PageOne imports StartPage, and so on). I'm also unsure if my example is simple enough, or if it could be simplified further.