Update label every time you click a button

Question

I’m using the Tkinter library for the program interface, so on the screen I would have a label that starts with your text represented by a variable of value '10', so at the click of a button, I wanted it to add 1 to the first variable, but he only does the desired the first time.

from tkinter  import * 
root = Tk()
root.geometry('500x500')

num = 10
def soma(a, b):                   
    s = a + b
    num = s
    lb['text'] = num

def bt_click():
    soma(num, 1)

lb = Label(root, text = f'{num}')
lb.place(x = 245, y = 235)

bt = Button(root, text = '+ 1', command = bt_click) 
bt.place(x = 175, y = 235)


root.mainloop()

Answer 1

In Python the variables referenced only within a function are implicitly global.

However if a variable has a value assigned anywhere in the body of the function, it will be assumed as local unless it is explicitly stated as global.

So if you want to assign value to a global variable within a local scope use the keyword global before using the variable.

def soma(a, b):   
    global num    # informa explicitamente ao interpretador que será utilizado uma variável global dentro desse escopo           
    num = a + b   # a atribuição agora é feita na variável global.
    lb['text'] = num

Answer 2

That function soma seems unnecessary to me, as you just update the contents of the button directly:

def bt_click():
    lb['text'] = int(lb['text']) + 1

Since the button text is a string, I use int to convert its contents to number, and then sum 1 and update the text of the button. So it looks like this:

from tkinter import *

root = Tk()
root.geometry('500x500')

def bt_click():
    lb['text'] = int(lb['text']) + 1

lb = Label(root, text = '10')
lb.place(x = 245, y = 235)

bt = Button(root, text = '+ 1', command = bt_click) 
bt.place(x = 175, y = 235)

root.mainloop()

---

If you want much use such a function soma, then I could do so:

def soma(a, b):
    lb['text'] = a + b

def bt_click():
    soma(int(lb['text']), 1)

I deleted the global variable because apparently there is no reason to keep it (but if there is, you can use the solution of the other answer to keep it updated). Also there is no reason to throw the sum in a variable s, only to then assign s label. If you are not going to use s for nothing else, delete it and play the result of the sum directly on the label.

Although the function soma should not update the button, it should only return the result of the sum and nothing else (and whoever calls the function does what they need with the result). I mean, something like that:

def soma(a, b):
    return a + b

def bt_click():
    lb['text'] = soma(int(lb['text']), 1)

But I still think I exaggerate for this particular case, and I would stick with the first solution I suggested, which doesn’t use the function soma. Of course, if you are going to do several other more complex things, then it is worth breaking into smaller functions. But for your specific case, I don’t think you need.

Answer 3

This happens because, within its method, the variable num is not the same as the one you stated. think that the num within the method only the method can see. To achieve this value, you must return it, that way:

def soma(a, b):                   
    s = a + b
    num = s
    lb['text'] = num
    return num

and to receive this number, in the method call, use:

num = soma(num, 1)