Jack Jack - 5 months ago 22
Python Question

Creating a backspace button on my calculator python tkinter GUI

i'm very new to python and i would like to fix the "C" button so that it clears the last number on the display. For example 321 would become 32, i have tried a lot of things but i can't seem to get it to work, i would be very grateful if someone could get it working, thank you. Here is the code:

from tkinter import *


class Calc():
def __init__(self):
self.total = 0
self.current = ""
self.new_num = True
self.op_pending = False
self.op = ""
self.eq = False


def num_press(self, num):
self.eq = False
temp = text_box.get()
temp2 = str(num)
if self.new_num:
self.current = temp2
self.new_num = False
else:
if temp2 == '.':
if temp2 in temp:
return
self.current = temp + temp2
self.display(self.current)

def calc_total(self):
self.eq = True
self.current = float(self.current)
if self.op_pending == True:
self.do_sum()
else:
self.total = float(text_box.get())

def display(self, value):
text_box.delete(0, END)
text_box.insert(0, value)

def do_sum(self):
if self.op == "add":
self.total += self.current
if self.op == "minus":
self.total -= self.current
if self.op == "times":
self.total *= self.current
if self.op == "divide":
self.total /= self.current
self.new_num = True
self.op_pending = False
self.display(self.total)

def operation(self, op):
self.current = float(self.current)
if self.op_pending:
self.do_sum()
elif not self.eq:
self.total = self.current
self.new_num = True
self.op_pending = True
self.op = op
self.eq = False

def cancel(self):
self.eq = False
self.current = "0"
self.display(0)
self.new_num = True

def all_cancel(self):
self.cancel()
self.total = 0

def sign(self):
self.eq = False
self.current = -(float(text_box.get()))
self.display(self.current)

sum1 = Calc()
root = Tk()
calc = Frame(root)
calc.grid()

root.title("Calculator")
text_box = Entry(calc, justify=RIGHT)
text_box.grid(row = 0, column = 0, columnspan = 3, pady = 5)
text_box.insert(0, "0")

numbers = "789456123"
i = 0
bttn = []
for j in range(1,4):
for k in range(3):
bttn.append(Button(calc, text = numbers[i]))
bttn[i].grid(row = j, column = k, pady = 5)
bttn[i]["command"] = lambda x = numbers[i]: sum1.num_press(x)
i += 1

bttn_0 = Button(calc, text = "0")
bttn_0["command"] = lambda: sum1.num_press(0)
bttn_0.grid(row = 4, column = 1, pady = 5)

bttn_div = Button(calc, text = chr(247))
bttn_div["command"] = lambda: sum1.operation("divide")
bttn_div.grid(row = 1, column = 3, pady = 5)

bttn_mult = Button(calc, text = "x")
bttn_mult["command"] = lambda: sum1.operation("times")
bttn_mult.grid(row = 2, column = 3, pady = 5)

minus = Button(calc, text = "-")
minus["command"] = lambda: sum1.operation("minus")
minus.grid(row = 3, column = 3, pady = 5)

point = Button(calc, text = ".")
point["command"] = lambda: sum1.num_press(".")
point.grid(row = 4, column = 0, pady = 5)

add = Button(calc, text = "+")
add["command"] = lambda: sum1.operation("add")
add.grid(row = 4, column = 3, pady = 5)

neg= Button(calc, text = "+/-")
neg["command"] = sum1.sign
neg.grid(row = 5, column = 0, pady = 5)

clear = Button(calc, text = "C")
clear["command"] = sum1.cancel
clear.grid(row = 5, column = 1, pady = 5)

all_clear = Button(calc, text = "AC")
all_clear["command"] = sum1.all_cancel
all_clear.grid(row = 5, column = 2, pady = 5)

equals = Button(calc, text = "=")
equals["command"] = sum1.calc_total
equals.grid(row = 5, column = 3, pady = 5)

root.mainloop()

Answer

Using a variable class is very handy for your situation because it will allow you to update the text of text_box using by setting the content of this variable. Note that doing so, you will not have to change anything else in your code apart from what I mention below:

var = StringVar() # Add this line
text_box = Entry(calc, justify=RIGHT, textvariable=var) # Modification here
text_box.grid(row = 0, column = 0, columnspan = 3, pady = 5)
var.set(0) # When you lunch the GUI, you will get 0 in text_box
#text_box.insert(0, "0") <-- You do not need this line

Now you will need to change the code cancel() as follows:

def cancel(self):
    global var
    self.eq = False
    self.current = self.current[:-1] # Remove last digit
    if self.current: # If there is at list one digit
       var.set(self.current)
    else: # In this case display 0 in text_box
       var.set(0)
    self.new_num = True

Unlike what other answers mentioned, you will need to remove self.display(0) in cancel().

Demo:

Let us type 123 and remove all the digits one by one:

enter image description here

Remove 3:

enter image description here

Remove 2:

enter image description here

Remove 1 (and here is where the if ... else condition above is useful):

enter image description here