How to create a child window and communicate with parents in Tkinter?

Tkinter provides powerful features for creating multi-window applications. You can create child windows (also called dialog boxes) that communicate with their parent windows to share data and respond to user interactions.

A child window in Tkinter is created using the Toplevel() constructor, which creates a new top-level window that can interact with its parent window.

Basic Child Window Creation

Here's how to create a simple child window and pass data from parent to child ?

import tkinter as tk
from tkinter import ttk

# Create main window
root = tk.Tk()
root.geometry("400x200")
root.title("Parent Window")

def open_child_window():
    # Create child window
    child = tk.Toplevel(root)
    child.geometry("300x150")
    child.title("Child Window")
    
    # Get data from parent and display in child
    user_input = entry.get()
    tk.Label(child, text=f"You entered: {user_input}", 
             font=('Arial', 12)).pack(pady=20)

# Create widgets in parent window
tk.Label(root, text="Enter some text:", font=('Arial', 11)).pack(pady=10)
entry = ttk.Entry(root, width=30)
entry.pack(pady=5)

button = ttk.Button(root, text="Open Child Window", command=open_child_window)
button.pack(pady=20)

root.mainloop()

Two-Way Communication Example

This example shows how to send data from child back to parent window ?

import tkinter as tk
from tkinter import ttk

class ParentChildApp:
    def __init__(self):
        self.root = tk.Tk()
        self.root.geometry("400x250")
        self.root.title("Parent Window")
        
        # Parent window widgets
        tk.Label(self.root, text="Parent Window", 
                font=('Arial', 14, 'bold')).pack(pady=10)
        
        self.parent_entry = ttk.Entry(self.root, width=30)
        self.parent_entry.pack(pady=5)
        
        ttk.Button(self.root, text="Send to Child", 
                  command=self.open_child).pack(pady=10)
        
        self.result_label = tk.Label(self.root, text="Result from child will appear here",
                                   fg='blue', font=('Arial', 10))
        self.result_label.pack(pady=10)
    
    def open_child(self):
        # Create child window
        self.child = tk.Toplevel(self.root)
        self.child.geometry("350x200")
        self.child.title("Child Window")
        
        # Display data from parent
        parent_data = self.parent_entry.get()
        tk.Label(self.child, text=f"From parent: {parent_data}",
                font=('Arial', 11)).pack(pady=10)
        
        # Child input widget
        tk.Label(self.child, text="Enter response:").pack(pady=5)
        self.child_entry = ttk.Entry(self.child, width=25)
        self.child_entry.pack(pady=5)
        
        # Button to send data back to parent
        ttk.Button(self.child, text="Send to Parent", 
                  command=self.send_to_parent).pack(pady=10)
        
        ttk.Button(self.child, text="Close", 
                  command=self.child.destroy).pack(pady=5)
    
    def send_to_parent(self):
        # Get data from child and update parent
        child_data = self.child_entry.get()
        self.result_label.config(text=f"Child sent: {child_data}")
        self.child.destroy()
    
    def run(self):
        self.root.mainloop()

# Run the application
app = ParentChildApp()
app.run()

Key Features

Modal Dialog Example

Creating a modal dialog that blocks interaction with the parent ?

import tkinter as tk
from tkinter import ttk

def create_modal_dialog():
    # Create modal dialog
    dialog = tk.Toplevel(root)
    dialog.geometry("300x150")
    dialog.title("Modal Dialog")
    
    # Make it modal (blocks parent interaction)
    dialog.grab_set()
    dialog.transient(root)  # Keep on top of parent
    
    tk.Label(dialog, text="This is a modal dialog", 
             font=('Arial', 12)).pack(pady=20)
    
    def close_dialog():
        dialog.grab_release()  # Release modal grab
        dialog.destroy()
    
    ttk.Button(dialog, text="Close", command=close_dialog).pack(pady=10)

# Create main window
root = tk.Tk()
root.geometry("400x200")
root.title("Main Window")

tk.Label(root, text="Click button to open modal dialog", 
         font=('Arial', 12)).pack(pady=30)

ttk.Button(root, text="Open Modal Dialog", 
           command=create_modal_dialog).pack(pady=20)

root.mainloop()

Conclusion

Use Toplevel() to create child windows that can communicate with their parent. For modal behavior, use grab_set() and transient() to control user interaction flow between windows.