Code from Tutorial

import tkinter as tk
from tkinter import ttk  # Think of this like the CSS for Tkinter

# Defining constants
LARGE_FONT = ("Century Gothic", 40)

# The 'tk.Tk' inside the parenthesis is inheritance. Meaning that the class 'VATest' will inherit from 'tk.Tk'
class VATest(tk.Tk):

    # This method will run everytime an instance of this class is initialized.
    # The 'self' parameter should always be passed through at the beginning. (Optional convention)
    # The '*args' and '**kwargs' are also both optional but conventionally included.
    # '*args' are used when passing through variables and '**kwargs' are used for dictionaries.
    def __init__(self, *args, **kwargs):

        # Initialize Tkinter
        tk.Tk.__init__(self, *args, **kwargs)

        # Set window title
        tk.Tk.wm_title(self, "Visual Acuity Assessment Tool")

        # Add container/window
        container = tk.Frame(self)

        # Putting elements inside the container
        container.pack(side="top", fill="both", expand=True)

        # First parameter is the minimum size. Weight is the priority.
        container.grid_rowconfigure(0, weight=1)
        container.grid_columnconfigure(0, weight=1)

        # Dictionary will contain the name of all frames.
        self.frames = {}

        for F in (StartPage, PageOne, PageTwo):
            frame = F(container, self)

            self.frames[F] = frame

            # Another option for arranging elements in a container. Pack or Grid
            # Sticky can be thought of as alignment and stretch.
            frame.grid(row=0, column=0, sticky="nsew")

        self.show_frame(StartPage)

    def show_frame(self, cont):

        # Select a user-defined frames inside the 'self.frames' dictionary
        # 'cont' serves as the controller to which frame is selected.
        frame = self.frames[cont]

        # Raise the selected frame from the dictionary.
        # I can use the 'tkraise' method because this class inherits from 'tk.Tk'.
        frame.tkraise()


class StartPage(tk.Frame):

    # Initialize the frame
    def __init__(self, parent, controller):

        # Initialize 'tk.Frame'
        # THe 'parent' class is the 'VATest' class
        tk.Frame.__init__(self, parent)

        # Adding text to a tkinter window
        label = tk.Label(self, text="StartPage", font=LARGE_FONT)
        label.pack(pady=10, padx=10)

        # Use 'lambda' with the 'command' parameter to create a nameless function within a single line.
        gotoPageOne = ttk.Button(
            self,
            text="Visit Page 1",
            command=lambda: controller.show_frame(PageOne),
        )
        gotoPageOne.pack()

        gotoPageTwo = ttk.Button(
            self,
            text="Visit Page 2",
            command=lambda: controller.show_frame(PageTwo),
        )
        gotoPageTwo.pack()


class PageOne(tk.Frame):
    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)

        label = tk.Label(self, text="PageOne", font=LARGE_FONT)
        label.pack(pady=10, padx=10)

        goHome = ttk.Button(
            self,
            text="Back to StartPage",
            command=lambda: controller.show_frame(StartPage),
        )
        goHome.pack()

        gotoPageTwo = ttk.Button(
            self,
            text="Visit Page 2",
            command=lambda: controller.show_frame(PageTwo),
        )
        gotoPageTwo.pack()


class PageTwo(tk.Frame):
    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)

        label = tk.Label(self, text="PageTwo", font=LARGE_FONT)
        label.pack(pady=10, padx=10)

        goHome = ttk.Button(
            self,
            text="Back to StartPage",
            command=lambda: controller.show_frame(StartPage),
        )
        goHome.pack()

        gotoPageOne = ttk.Button(
            self,
            text="Visit Page 1",
            command=lambda: controller.show_frame(PageOne),
        )
        gotoPageOne.pack()


app = VATest()
app.mainloop()