Python Tkinter update when entry is changed

Question

I'm trying to make a simple temperature conversion calculator in python. What I want to do is to be able to type in a number, and have the other side automatically update, without having to push a button. Right now I can only get it to work in one direction. I can either code it so that it can go from F to C, or C to F. But not either way.

Obviously after is not the way to go. I need some kind of onUpdate or something. TIA!

import Tkinter as tk

root = tk.Tk()
temp_f_number = tk.DoubleVar()
temp_c_number = tk.DoubleVar()

tk.Label(root, text="F").grid(row=0, column=0)
tk.Label(root, text="C").grid(row=0, column=1)

temp_f = tk.Entry(root, textvariable=temp_f_number)
temp_c = tk.Entry(root, textvariable=temp_c_number)

temp_f.grid(row=1, column=0)
temp_c.grid(row=1, column=1)

def update():
    temp_f_float = float(temp_f.get())
    temp_c_float = float(temp_c.get())

    new_temp_c = round((temp_f_float - 32) * (5 / float(9)), 2)
    new_temp_f = round((temp_c_float * (9 / float(5)) + 32), 2)

    temp_c.delete(0, tk.END)
    temp_c.insert(0, new_temp_c)

    temp_f.delete(0, tk.END)
    temp_f.insert(0, new_temp_f)

    root.after(2000, update)

root.after(1, update)
root.mainloop()

Answer 1

What you are looking for is variable trace() method. E.g.:

def callback(*args):
    print "variable changed!"

var = DoubleVar()
var.trace("w", callback)

Attach trace callbacks for each of your DoubleVar, for temp_f_number one to update the temp_c_number value and vice versa. You'll likely also need to disable one callback function while inside another one, to avoid recursive update cycle.

Another note - do not edit the Entry fields. Instead, use variables' set() method. Entry fields will be updated automatically.

So, complete code could look like this:

import Tkinter as tk

root = tk.Tk()
temp_f_number = tk.DoubleVar()
temp_c_number = tk.DoubleVar()

tk.Label(root, text="F").grid(row=0, column=0)
tk.Label(root, text="C").grid(row=0, column=1)

temp_f = tk.Entry(root, textvariable=temp_f_number)
temp_c = tk.Entry(root, textvariable=temp_c_number)

temp_f.grid(row=1, column=0)
temp_c.grid(row=1, column=1)

update_in_progress = False

def update_c(*args):
    global update_in_progress
    if update_in_progress: return
    try:
        temp_f_float = temp_f_number.get()
    except ValueError:
        return
    new_temp_c = round((temp_f_float - 32) * 5 / 9, 2)
    update_in_progress = True
    temp_c_number.set(new_temp_c)
    update_in_progress = False

def update_f(*args):
    global update_in_progress
    if update_in_progress: return
    try:
        temp_c_float = temp_c_number.get()
    except ValueError:
        return
    new_temp_f = round(temp_c_float * 9 / 5 + 32, 2)
    update_in_progress = True
    temp_f_number.set(new_temp_f)
    update_in_progress = False

temp_f_number.trace("w", update_c)
temp_c_number.trace("w", update_f)

root.mainloop()

Answer 2

.trace is soon to be deprecated, use .trace_add instead:

var = tk.StringVar()
var.trace_add('write', callback)

Same functionality, but you must pass write or read instead of w or r.