function as argument for callback-function with tkinter-CodePudding

I am using SQLite and tkinter in Python 3.9 and got the following problem:

I got 1 SQLite DB with 2 tables. I am using the coloumn of one of the tables for a tkinter option menu. Now I want to trace the variable selected and display a corresponding value from the other table on a tk.label. OptionMenu and label are in the same window.

This is what I tried but returns: TypeError: 'NoneType' object is not callable

def callBackFct(var):
        cur.execute("""SELECT nameFromColoumn FROM sqlDbTable WHERE condition = ?""", (var,))
        f = cur.fetchall()
        tkLabel1.config(text = f[0][0])

optionMenuVar1.trace("w", callBackFct(optionMenuVar1.get()))

If I let the function get the variable like this...

def callBackFct(*args):
        var = optionMenuVar1.get() # getting variable inside function
        cur.execute("""SELECT nameFromColoumn FROM sqlDbTable WHERE condition = ?""", (var,))
        f = cur.fetchall()
        tkLabel1.config(text = f[0][0])

optionMenuVar1.trace("w", callBackFct))

it works, but I have 50 labels and dont want to copy paste 50 functions. There hast to be a more elegant solution right, where I can give an argument to the callback function?! Please help. It seems like I have missunderstood something basic...

CodePudding user response：

Every function in python needs to return something. So when you do this callBackFct(optionMenuVar1.get()) callBackFct returns None since it returns nothing else. lambda is a way to store a function with provided arguments and is not executed when declared.

You can use an annonymous function for this case.

import tkinter as tk
def test(x):
    print(x)
root = tk.Tk()
var = tk.StringVar()
opt = tk.OptionMenu(root,var,value='hi')
opt.pack()
var.trace('w',lambda *args:test(var.get()))

root.mainloop()

Another maybe easier way to understand is to follow this Tutorial/Documentation here. Which could lead to the following code for you:

def callBackFct(var,idx,mod):
        cur.execute("""SELECT nameFromColoumn FROM sqlDbTable WHERE condition = ?""", (var.get(),))
        f = cur.fetchall()
        tkLabel1.config(text = f[0][0])

optionMenuVar1.trace("w", callBackFct))