Tkinter Entry widget requires two events to update

Question

I've been trying to develop a small gui to calculate some totals with weighting etc. While trying to get a total to update real-time with a changing entry, I noticed it took two events to update; This is a simplified version of my code that shows the problem:

from Tkinter import *
root=Tk()
frame=Frame(root)
frame.pack()
entry=Entry(frame)
entry.pack()
label=Label(frame,text="entry:")
label.pack()
def updatelabel(event):
    label=Label(frame,text="entry:"+entry.get())
    label.pack()
    print "called"
entry.bind("<Key>", updatelabel)

root.mainloop()

When you input into the field the function calls, but does not update to what was typed until the next character is typed. How would I go about getting the label to update to what is in the field at the time?

Creating a new `Label` every time the user presses a key is not the way to go. Look into `StringVar` and variable tracing. — TigerhawkT3, Aug 30 '15 at 08:04
I'm aware, I just threw it together quickly to demonstrate the problem :) — John Doe, Aug 30 '15 at 08:05
This answer explains why it _seems_ to require two events: http://stackoverflow.com/a/3513906/7432 — Bryan Oakley, Aug 30 '15 at 13:55

score 1 · Answer 1 · answered Aug 30 '15 at 09:29

1

You don't really need to explicitly process events and use callback functions to accomplish what you want. In other words it's possible to get Tkinter to do it for you automatically using a StringVar().

from Tkinter import *

root=Tk()
frame=Frame(root)
frame.pack()

entry_var = StringVar()
entry_var.set('')

entry = Entry(frame, textvariable=entry_var)
entry.pack()

label = Label(frame, text='entry: ')
label.pack(side=LEFT)

contents = Label(frame, textvariable=entry_var)
contents.pack(side=LEFT)

entry.focus_set()  # force initial keyboard focus to be on entry widget
root.mainloop()

answered Aug 30 '15 at 09:29

martineau

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I didn't know about [textvariable] martineau, very neat! Thank you. – Reblochon Masque Aug 30 '15 at 09:39
1

@Reblochon: Here's some more [information on them](http://effbot.org/tkinterbook/variable.htm). There;s also [this reference](http://infohost.nmt.edu/tcc/help/pubs/tkinter/web/control-variables.html) which explains how they work in conjunction with other widgets, like `Label` and `Entry`. – martineau Aug 30 '15 at 09:54

Reblochon Masque · Answer 2 · 2015-08-30T09:24:59.250

No it doesn't require two entries to be called, it is called on the first entry. The key bindings are on the Entry widgets to avoid the problems that a binding to the root will create if you have more than one entry widget.

import tkinter as tk

class SmallApp(tk.Frame):
    def __init__(self, master = None):
        tk.Frame.__init__(self, master)
        self.master = master
        self.pack()
        self.entry = tk.Entry(self)
        self.entry.pack()
        self.var = "entry:"
        self.label = tk.Label(text = self.var)
        self.label.pack()
        self.entry.bind("<Key>", self.updatelabel)

    def updatelabel(self, event):
        self.var += event.char
        self.label.configure(text=self.var)

root = tk.Tk()
app = SmallApp(root)
app.mainloop()

It does seem like the output of .get is buffered until the next event, as using StringVar exhibits the same properties. Any way of forcing an event to be called that would update it? — John Doe, Aug 30 '15 at 08:28

score 0 · Accepted Answer · answered Aug 30 '15 at 08:45

Instead of using entry.bind("<Key>", update label), I used the root window instead: root.bind("<Key>", update label). This did the trick, however it is important to realize that the function updatelabel() will be called every time a key is pressed in your tkinter window. This could cause some problems if you have more than one entry box updating labels.

Here is the code I wrote with a few modifications:

from Tkinter import *

root=Tk()
frame=Frame(root)
frame.pack()
update_label = StringVar()  # Made a StringVar so you don't get new labels every time a key is pressed.
update_label.set("entry:")
entry=Entry(frame)
entry.pack()

label=Label(frame,textvariable=update_label)  # Used textvariable= instead of text=
label.pack()

def updatelabel(event):
    update_label.set("entry:" + entry.get())  # Setting the variable used in textvariable=
    print "called"

root.bind("<Key>", updatelabel)  # Changed entry.bind to root.bind

root.mainloop()

Thanks, that worked! I assume passing between the frame, entry and root delay it at some point. — John Doe, Aug 30 '15 at 08:48

Tkinter Entry widget requires two events to update

3 Answers3