BRIEF EXPLANATION

1-2 : Imports

Imports the tkinter library for creating the GUI (Graphical User Interface)
font module allows custom fonts (though not heavily used here)

4-20 : The def init (self)

This runs automatically when you create a calculator. It:

• Creates the main window
  
• Sets up variables to track the calculator's state
  
• Calls helper methods to build the UI

Key variables:

• self.expression = "" - stores the calculation being built (e.g., "5+3")
  
• self.result_text - displays the current number
  
• self.expression_text - shows the calculation history