Multiple windows (TopLevel widgets)

Difference between Tk and Toplevel

Tk is the absolute root of the application, it is the first widget that needs to be instantiated and the GUI will shut down when it is destroyed.

Toplevel is a window in the application, closing the window will destroy all children widgets placed on that window{1} but will not shut down the program.

try:
    import tkinter as tk #python3
except ImportError:
    import Tkinter as tk #python2

#root application, can only have one of these.
root = tk.Tk() 

#put a label in the root to identify the window.
label1 = tk.Label(root, text="""this is root
closing this window will shut down app""")
label1.pack()

#you can make as many Toplevels as you like
extra_window = tk.Toplevel(root)
label2 = tk.Label(extra_window, text="""this is extra_window
closing this will not affect root""")
label2.pack()

root.mainloop()

If your python program only represents a single application (which it almost always will) then you should have only one Tk instance, but you may create as many Toplevel windows as you like.

try:
    import tkinter as tk #python3
except ImportError:
    import Tkinter as tk #python2

def generate_new_window():
    window = tk.Toplevel()
    label = tk.Label(window, text="a generic Toplevel window")
    label.pack()

root = tk.Tk()

spawn_window_button = tk.Button(root,
                                text="make a new window!",
                                command=generate_new_window)
spawn_window_button.pack()

root.mainloop()

{1}: if a Toplevel (A = Toplevel(root)) is the parent of another Toplevel (B = Toplevel(A)) then closing window A will also close window B.

arranging the window stack (the .lift method)

The most basic case to lift a particular window above the others, just call the .lift() method on that window (either Toplevel or Tk)

import tkinter as tk #import Tkinter as tk #change to commented for python2

root = tk.Tk()

for i in range(4):
    #make a window with a label
    window = tk.Toplevel(root)
    label = tk.Label(window,text="window {}".format(i))
    label.pack()
    #add a button to root to lift that window
    button = tk.Button(root, text = "lift window {}".format(i), command=window.lift)
    button.grid(row=i)

root.mainloop()

However if that window is destroyed trying to lift it will raise an error like this:

Exception in Tkinter callback
Traceback (most recent call last):
  File "/.../tkinter/__init__.py", line 1549, in __call__
    return self.func(*args)
  File "/.../tkinter/__init__.py", line 785, in tkraise
    self.tk.call('raise', self._w, aboveThis)
_tkinter.TclError: bad window path name ".4385637096"

Often when we are trying to put a particular window in front of the user but it was closed a good alternative is to recreate that window:

import tkinter as tk #import Tkinter as tk #change to commented for python2

dialog_window = None

def create_dialog():
    """creates the dialog window
  ** do not call if dialog_window is already open, this will
     create a duplicate without handling the other
if you are unsure if it already exists or not use show_dialog()"""
    global dialog_window
    dialog_window = tk.Toplevel(root)
    label1 = tk.Label(dialog_window,text="this is the dialog window")
    label1.pack()
    #put other widgets
    dialog_window.lift() #ensure it appears above all others, probably will do this anyway

def show_dialog():
    """lifts the dialog_window if it exists or creates a new one otherwise"""
    #this can be refactored to only have one call to create_dialog()
    #but sometimes extra code will be wanted the first time it is created
    if dialog_window is None:
        create_dialog()
        return
    try:
        dialog_window.lift()
    except tk.TclError:
        #window was closed, create a new one.
        create_dialog()
    
    
root = tk.Tk()

dialog_button = tk.Button(root,
                          text="show dialog_window",
                          command=show_dialog)
dialog_button.pack()
root.mainloop()

This way the function show_dialog will show the dialog window whether it exists or not, also note that you can call .winfo_exists() to check if it exists before trying to lift the window instead of wrapping it in a try:except.

There is also the .lower() method that works the same way as the .lift() method, except lowering the window in the stack:

import tkinter as tk #import Tkinter as tk #change to commented for python2

root = tk.Tk()
root.title("ROOT")
extra = tk.Toplevel()
label = tk.Label(extra, text="extra window")
label.pack()

lower_button = tk.Button(root,
                         text="lower this window",
                         command=root.lower)
lower_button.pack()

root.mainloop()

You will notice that it lowers even below other applications, to only lower below a certain window you can pass it to the .lower() method, similarly this can also be done with the .lift() method to only raise a window above another one.