The Tk library actually has a function that does this although it is nominally 'private'. You can use it as follows.
import tkinter as tk
root = tk.Tk()
root.wm_geometry("800x600")
dialog = tk.Toplevel(root)
root_name = root.winfo_pathname(root.winfo_id())
dialog_name = dialog.winfo_pathname(dialog.winfo_id())
root.tk.eval('tk::PlaceWindow {0} widget {1}'.format(dialog_name, root_name))
root.mainloop()
This will place your dialog centred over the specified window (in this case the root window). Ref.
In a real life application
Here's a version I made because in real life you'll need to show an InputBox over the top of the Main Window/Form, and typically in a Modal (can't click on other windows) state, and then close it when the user clicks OK:
try:
# for Python2
import Tkinter as tk
except ImportError:
# for Python3
import tkinter as tk
class App:
def __init__(self):
self.HEIGHT = 700
self.WIDTH = 800
root = tk.Tk()
root.width = self.WIDTH
root.height = self.HEIGHT
self.dialogroot = root
self.strDialogResult = ""
self.canvas = tk.Canvas(root, height=self.HEIGHT, width=self.WIDTH)
self.canvas.pack()
frame = tk.Frame(root, bg='#42c2f4')
frame.place(relx=0.5, rely=0.02, relwidth=0.96, relheight=0.95, anchor='n')
# Here is the button call to the InputBox() function
buttonInputBox = tk.Button(frame, text="Input Box", bg='#cccccc', font=60,
command=lambda: self.InputBox())
buttonInputBox.place(relx=0.05, rely=0.1, relwidth=0.90, relheight=0.8)
root.mainloop()
def InputBox(self):
dialog = tk.Toplevel(self.dialogroot)
dialog.width = 600
dialog.height = 100
frame = tk.Frame(dialog, bg='#42c2f4', bd=5)
frame.place(relwidth=1, relheight=1)
entry = tk.Entry(frame, font=40)
entry.place(relwidth=0.65, rely=0.02, relheight=0.96)
entry.focus_set()
submit = tk.Button(frame, text='OK', font=16, command=lambda: self.DialogResult(entry.get()))
submit.place(relx=0.7, rely=0.02, relheight=0.96, relwidth=0.3)
root_name = self.dialogroot.winfo_pathname(self.dialogroot.winfo_id())
dialog_name = dialog.winfo_pathname(dialog.winfo_id())
# These two lines show a modal dialog
self.dialogroot.tk.eval('tk::PlaceWindow {0} widget {1}'.format(dialog_name, root_name))
self.dialogroot.mainloop()
#This line destroys the modal dialog after the user exits/accepts it
dialog.destroy()
#Print and return the inputbox result
print(self.strDialogResult)
return self.strDialogResult
def DialogResult(self, result):
self.strDialogResult = result
#This line quits from the MODAL STATE but doesn't close or destroy the modal dialog
self.dialogroot.quit()
# Launch ...
if __name__ == '__main__':
app = App()