Weird error in pycharm

[TheRedFedora]

Hello everyone,
I had the plan to make a calculator for my schoolproject. Everything I did seemed fine until the point that I actually ran the whole thing. Then the error occurred.

Error:
line 33, in equate
    self.result = float(self.var1) - float(self.var2)
ValueError: could not convert string to float: 

this is my code:

from tkinter import *

master = Tk()
display = Entry(master, width=20, justify='right', bd=0, bg='lightgrey')

master.title("calculator")

class Calculator:

    def __init__ (self):
        self.var1 = ""
        self.var2 = ""
        self.result = 0
        self.current = 0
        self.operator = 0

    def number_button(self, index):
        if self.current is 0:
            self.var1 = str(self.var1) + str(index)
            display.delete(0, END)
            display.insert(0,string=self.var1)
        else:
            self.var1 = str(self.var2) + str(index)
            display.delete(0, END)
            display.insert(0, string=self.var2)



    def equate(self):
        if self.operator is 0:
            self.result = float(self.var1) + float(self.var2)
        elif self.operator is 1:
            self.result = float(self.var1) - float(self.var2)
        elif self.operator is 2:
            self.result = float(self.var1) * float(self.var2)
        elif self.operator is 3:
            self.result = float(self.var1) / float(self.var2)
        display.delete(0, END)
        display.insert(0, string=self.result)

    def set_op(self, op):
        self.operator = op
        display.delete(0, END)
        if self.current is 0:
            self.current = 1
        else:
            self.equate()
            self.var2=""

    def clear(self):
        self.__init__()
        display.delete(0, END)

calc = Calculator()

b0 = Button(master, text="0", command=lambda: calc.number_button(0))
b1 = Button(master, text="1", command=lambda: calc.number_button(1))
b2 = Button(master, text="2", command=lambda: calc.number_button(2))
b3 = Button(master, text="3", command=lambda: calc.number_button(3))
b4 = Button(master, text="4", command=lambda: calc.number_button(4))
b5 = Button(master, text="5", command=lambda: calc.number_button(5))
b6 = Button(master, text="6", command=lambda: calc.number_button(6))
b7 = Button(master, text="7", command=lambda: calc.number_button(7))
b8 = Button(master, text="8", command=lambda: calc.number_button(8))
b9 = Button(master, text="9", command=lambda: calc.number_button(9))
b_dot = Button(master, text=".", command=lambda: calc.number_button("."))

plus = Button(master, text="+",  command=lambda: calc.set_op(0))
minus = Button(master, text="-",  command=lambda: calc.set_op(1))
times = Button(master, text="x",  command=lambda: calc.set_op(2))
divide = Button(master, text="/",  command=lambda: calc.set_op(3))

equals = Button (master, text="=", command=calc.equate)
clear = Button (master, text="c", command=calc.clear)


display.place(x=0, y=2)
b7.grid(row=1, column=0)
b8.grid(row=1, column=1)
b9.grid(row=1, column=2)
b4.grid(row=2, column=0)
b5.grid(row=2, column=1)
b6.grid(row=2, column=2)
b1.grid(row=3, column=0)
b2.grid(row=3, column=1)
b3.grid(row=3, column=2)
b0.grid(row=4, column=0)
b_dot.grid(row=4, column=1)
clear.grid(row=4, column=2)

plus.grid(row=1, column=3)
minus.grid(row=2, column=3)
times.grid(row=3, column=3)
divide.grid(row=4, column=3)
equals.grid(row=4, column=3)
master.mainloop()

The problem seems to be coming out of the "equate" section. Is there a way to fix it or is there maybe an alternative code that would do the trick?

*Quick edit, the GUI is working, the error occurres after I put in my first variable and press on any of the options from equate. Then it doesn't type numbers anymore and gives an error when I hit the "="

TheRedFedora

[Larz60+]

That is not a wierd error, it's telling you that either var1 or var2 is not a valid string for conversion
just prior to line 33, on same indentation level, add

            print('var1: {}, type(var1): {}, var2: {}, type var2: {}'.format(var1, type(var1), var2, type(var2)))
            self.result = float(self.var1) - float(self.var2)

I think your problem will become evident.
during development, whenever you get an error like this, your first action should be to display the inputs and outputs.