Test program returns int (I want int) and real one returns tuple (not wanted)

[buran]

Everything is important!

Actually, your original problem can be reproduced with 13 lines of code. At that point (and probably long before that) you would have spotted the problem.

from tkinter import *

class SearchWin:
    def searchList(self):
        elements = int(itembox.size())
             
    def __init__(self):
        self.searchPrompt = Tk()
        self.searchPrompt.geometry("150x150")
        searchbtn = Button(self.searchPrompt, text = "Search", command = self.searchList)
        searchbtn.pack()
 

s=SearchWin()
itembox = Entry(width = 50)
itemslist = Listbox(listvariable = itemsvar)
s.searchPrompt.mainloop()

Note, I used your original code with very small modifications. In any case you have big problem with overall structure of your code