How to add class variable values to a list, python 3x?

I'm not a big Tkinter user, but I have battled with it enough in the past to get your code working!

from tkinter import *
from tkinter import ttk

# Create an instance of tkinter frame or window
win = Tk()

# Set the size of the window
win.geometry("700x350")



class ComboWidget():
    def __init__(self):
        
        # Create a combobox widget
        self.var = StringVar()
        self.cb = ttk.Combobox(win, textvariable= self.var)
        # Define  Tuple
        self.data = [('First', 'F', '0.5'), ('Next', 'N', '1.0'), ('Middle', 'M', '0.6'), ('Last', 'L', '0.24')]

        self.cb['values'] = self.data
        self.cb['state'] = 'readonly'
        self.cb.pack(fill = 'x',padx=5, pady=5)
        
       
        # 1. Commented out the declaration.
        self.value_list = []

        self.cb.bind("<<ComboboxSelected>>", self.handle_selection)
    
        # Create a button to clear the selected combobox text value
        self.button = Button(win, text= "Clear", command= self.clear_cb)
        self.button.pack()

    # Create a function to clear the combobox
    def clear_cb(self):
       self.cb.set('')
       
    def handle_selection(self, _):
        selected_index = self.cb.current()  # Get the index of the selected item
        print(selected_index)
        selected_tuple = self.data[selected_index]  # Get the selected tuple
        print(selected_tuple)
        selected_float = float(selected_tuple[-1])  # Extract the float value from the tuple
        print(selected_float)  # Print the extracted float value
        
        # 2. Commented out these lines:
        self.value_list.append(selected_float)
        print('The value_list value is: ')
        print(self.value_list)

c=ComboWidget()
win.mainloop()

It could still use some tidying up, but I hope this gets you back on track. Here were some of the issues:

1. As @meshkati commented, you actually have to instantiate ComboWidget.

2. As @OneCricketeer commented, the two functions you defined were actually written as methods of ComboWidget and therefore belong in the body of the class.

3. Your definition of self.data needs to precede its first use

4. handle_selection and clear_cb needed to be prefixed by 'self.' where they are bound to methods

5. The self.handle_selection method should have 2 paramaters: self because it is a method, and an event. Because you don't actually use the event passed in, you can just put '_'

I think that's it - good luck with your further development of this project!