Intercept event when combobox edited

Question

I'm using a ComboBox as part of a larger GUI, written in python/tkinter.

When the drop-down selection is changed, the color is updated to indicate to the user that something has changed.

However, the combobox also allows the user to type in their own value. I also want the color to change when this happens.

The problem is, I don't see anything in the ComboBox documentation indicating how to do this.

Solution 1

I noticed that somewhere in Python's Tkinter docs, it mentions that Combobox is a subclass of Entry.

With an Entry, here's what you do. Set the configuration:

• -validatecommand should be set to the thing that you want to happen when a key is pressed (in my case, change a color).
• -validate should be set to key, meaning that the validation command will be called whenever a key is pressed while the cursor is in the entry.

Here's the tk page on text entries for further reference.

Doing the exact same thing for comboboxes works just the same way (as far as I can tell).

Solution 2

You can use a StringVar as a parameter of the Combobox constructor. This StringVar can be traced (ie, subscribe to each change).

Here a small example:

from Tkinter import *
from ttk import *

def on_field_change(index, value, op):
    print "combobox updated to ", c.get()

root = Tk()
v = StringVar()
v.trace('w',on_field_change)
c = Combobox(root, textvar=v, values=["foo", "bar", "baz"])
c.pack()

mainloop()

Solution 3

Just bind '<<ComboboxSelected>>' to a method...

import tkinter as tk
from tkinter import ttk

class Main(tk.Tk):
     
  def __init__(self, *args, **kwargs):
    tk.Tk.__init__(self, *args, **kwargs)
    self.container = tk.Frame(self)
    self.container.pack(side="top", fill = "both", expand=True)
    self.container.grid_rowconfigure(0, weight=1)
    self.container.grid_columnconfigure(0, weight=1)
    self.cb=ttk.Combobox(self.container, values=[0,1, 2, 3] , state='readonly')
    self.cb.bind('<<ComboboxSelected>>', self.modified)    
    self.cb.pack()
                  
  def modified (self, event) :
      print(self.cb.get())
    
main = Main()
main.mainloop()