I can't access variables from one class in another one - Python

I'm learning OOP and am making a calculator. I have classes for the main program (the parent program), buttons, and entries.

Main program:

class Program(tk.Tk): 

    def __init__(self, title, size): 
        super().__init__()
        self.title(title)
        self.geometry(f"{size[0]}x{size[1]}")

        # Program elements split into widget type
        self.entries = Entries(self) 
        self.output = Output(self)
        self.buttons = Buttons(self)

Entries:

class Entries(ttk.Frame):
    
    def __init__(self, parent): 
        super().__init__(parent) 
        self.place(x = 0, y = 0) 

        # Calling widget generator and placer functions
        self.entryGen() 
        self.entryPlacer()
    
    def entryGen(self): # Create entry widgets
        self.firstTerm = ttk.Entry(self) 
        self.commonDifference = ttk.Entry(self)
        self.numberOfTerms = ttk.Entry(self)
    
    def entryPlacer(self): # Place entry widgets
        self.firstTerm.pack()
        self.commonDifference.pack()
        self.numberOfTerms.pack()

Buttons:

class Buttons(ttk.Frame):

    def __init__(self, parent):
        super().__init__(parent)
        self.place(x = 0, y = 0)

        self.buttonGen()
        self.buttonPlacer()

    def buttonGen(self):
        self.clear = ttk.Button(self, text = "Clear", command = lambda: self.clear)
        self.calculate = ttk.Button(self, text = "Calculate", command = lambda: self.calculate)
    
    def buttonPlacer(self):
        self.clear.pack()
        self.calculate.pack()
    
    def clear(self):
        pass

    def calculate(self):
        entries = Entries()
        try:
            print(entries.firstTerm.get() + entries.commonDifference.get() + entries.numberOfTerms.get()) # Just some test code to see if it works, which it doesnt
        except:
            pass

I expect that when I create a reference to the Entries() class in my Buttons() class, I can access the variables I made in the Entries() class. This is not the case and I can't figure out why. I have looked at similar fixes and none seem to work.

Solution

There are few issues in your code:

you should not create another instance of Entries inside Buttons.calculate(). Suggest to pass the instance to Buttons when it is created;
the function self.calculate() is not called actually in the expression command=lambda: self.calculate. Use command=self.calculate instead. Same issue for command=lambda: self.clear;
if the three inputs are numbers, you need to convert them to numbers before doing the calculation.

Updated code:

import tkinter as tk
from tkinter import ttk

class Program(tk.Tk):

    def __init__(self, title, size):
        super().__init__()
        self.title(title)
        self.geometry(f"{size[0]}x{size[1]}")

        # Program elements split into widget type
        self.entries = Entries(self)
        self.output = Output(self)
        self.buttons = Buttons(self, self.entries) ### pass self.entries to class Buttons

class Entries(ttk.Frame):

    def __init__(self, parent):
        super().__init__(parent)
        self.place(x = 0, y = 0)

        # Calling widget generator and placer functions
        self.entryGen()
        self.entryPlacer()

    def entryGen(self): # Create entry widgets
        self.firstTerm = ttk.Entry(self)
        self.commonDifference = ttk.Entry(self)
        self.numberOfTerms = ttk.Entry(self)

    def entryPlacer(self): # Place entry widgets
        self.firstTerm.pack()
        self.commonDifference.pack()
        self.numberOfTerms.pack()

class Buttons(ttk.Frame):

    def __init__(self, parent, entries): ### added argument entries
        super().__init__(parent)
        self.place(x = 200, y = 0)

        self.entries = entries  ### save the passed entries
        self.buttonGen()
        self.buttonPlacer()

    def buttonGen(self):
        self.clear = ttk.Button(self, text = "Clear", command = self.clear) ### remove lambda
        self.calculate = ttk.Button(self, text = "Calculate", command = self.calculate) ### remove lambda

    def buttonPlacer(self):
        self.clear.pack()
        self.calculate.pack()

    def clear(self):
        pass

    def calculate(self):
        try:
            ### convert inputs to numbers before calculation
            print(float(self.entries.firstTerm.get()) + float(self.entries.commonDifference.get()) + float(self.entries.numberOfTerms.get()))
        except Exception as ex:
            ### better show the execption
            print(ex)