Hey there! The issue here is that your IntVar starts with its default value of 0, which matches your offvalue—so the checkbutton stays unselected when your app launches. Let's fix this with two straightforward approaches:

Approach 1: Set the IntVar value during initialization #

You can pass the value parameter directly when creating your IntVar to set it to your onvalue (which is 1 in your code):

import tkinter as tk
from tkinter import ttk

root = tk.Tk()
# Initialize the variable with the onvalue (1) to make the checkbutton checked by default
v = tk.IntVar(value=1)
cb = ttk.Checkbutton(root, text='Remember user', onvalue=1, offvalue=0, variable=v)
cb.pack()

root.mainloop()

Approach 2: Use the set() method after creating the variable #

If you prefer to initialize the variable first and set its value later, call v.set(1) right after defining it:

import tkinter as tk
from tkinter import ttk

root = tk.Tk()
v = tk.IntVar()
# Set the variable to the onvalue to default to checked
v.set(1)
cb = ttk.Checkbutton(root, text='Remember user', onvalue=1, offvalue=0, variable=v)
cb.pack()

root.mainloop()

Why this works: #

The Checkbutton uses the value of its linked variable to determine its state. Since you set onvalue=1 and offvalue=0, setting the variable to 1 tells the checkbutton to render as checked as soon as the app starts.

内容的提问来源于stack exchange，提问作者Thomas Caio