How to return a value of a function given as a command in tkinter

Question

I've written a very simple code with python tkinter, it contains an input box. I'd like to keep the value inserted by the user to myself, in case I need to use it later.

Here's the code:

import tkinter as tk 
   
root=tk.Tk() 
  
root.geometry("600x400") 
   
def submit(): 
  
    name=name_entry.get() 
    return name
       
name_label = tk.Label(root, text = 'Username', 
                      font=('calibre', 
                            10, 'bold')) 
    
name_entry = tk.Entry(root, 
                      font=('calibre',10,'normal')) 
     
sub_btn=tk.Button(root,text = 'Submit', 
                  command = submit) 

name_label.grid(row=0,column=0) 
name_entry.grid(row=0,column=1)  
sub_btn.grid(row=2,column=1) 

root.mainloop()

In function submit I've written return name, in order to return the inserted name of the user. But how can I access it outside of the function? I want to keep the value somewhere, but I don't know how to.

I appreciate your kind help in advance

The simple way is using global variable to store the value. So add `global name` at the beginning of `submit()`. — acw1668, Nov 05 '20 at 11:56

Reblochon Masque · Accepted Answer · 2020-11-05T15:42:19.687

1

Most times, callbacks drop the return value (all tkinter callbacks do), if any. This is because is is a bit awkward to manage return values using the observer pattern which is most often used for callbacks.

There are two main ways to transfer a value from the CB to where it is needed:

One is to use a container object - that may be a simple python list or dict, or a more elaborate data class that is accessible by the callback, and mutated to assign the proper values to it.

here is a very simple example using a dictionary:

import tkinter as tk 

def print_info():
    print(f'from print_info: {information_transfer}')

def submit(): 
    information_transfer['name'] = name_entry.get() 
    print(f'after input: {information_transfer}')
       
root=tk.Tk()   
root.geometry("600x400") 
   
name_label = tk.Label(root, text = 'Username', font=('calibre', 10, 'bold')) 
name_entry = tk.Entry(root, font=('calibre', 10, 'normal')) 
     
sub_btn = tk.Button(root,text='Submit', command=submit)
info_btn = tk.Button(root,text='print info', command=print_info)


name_label.grid(row=0, column=0) 
name_entry.grid(row=0, column=1)  
sub_btn.grid(row=2, column=1) 
info_btn.grid(row=3, column=1) 

information_transfer = {'name': None, 'other': None}
print(f'before input: {information_transfer}')

root.mainloop()

Another is the OOP approach described by @DavidGildour in another answer.

edited Nov 05 '20 at 15:42

answered Nov 05 '20 at 12:21

Reblochon Masque

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1

By running your code I get the error ```name is not defined``` – Liana Nov 05 '20 at 13:45
Yes, of course, sorry - the dict keys should be strings; I corrected the typo. – Reblochon Masque Nov 05 '20 at 13:50
well now I don't get any errors, but printing ```information_transfer[name]``` shows me ```None```, so it seems like I'm still not having access to the user input. – Liana Nov 05 '20 at 13:54
You need to input something in the entry form, then press submit prior to printing; I added print statements: the first to be called will show `information_transfer` with `None` values; the second (in the `submit` function) will show the input from the `entry` form. – Reblochon Masque Nov 05 '20 at 13:58
Ah yes, sorry, I don't know why it didn't work on the first go, thank you so so much :)) – Liana Nov 05 '20 at 14:00
1

You are very welcome - I added a `print_info` function, and a button that calls it for your convenience. – Reblochon Masque Nov 05 '20 at 14:04
Perfect! I really appreciate it all :) – Liana Nov 05 '20 at 14:05
1

Not "most" times. Callbacks always throw away the return value. – Bryan Oakley Nov 05 '20 at 14:15
Agreed @BryanOakley , all `tkinter` callbacks do - I've added this important precision in my answer, thank you! – Reblochon Masque Nov 05 '20 at 15:43

score 0 · Answer 2 · edited Nov 05 '20 at 11:52

I would recommend using OOP approach, to have an object retaining a state:

import tkinter as tk


class Program:
    def __init__(self):
        self.root = tk.Tk()
        self.name_label = tk.Label(self.root, text='Username',
                                   font=('calibre', 10, 'bold'))
        self.name_entry = tk.Entry(self.root, font=('calibre', 10, 'normal'))
        self.sub_btn = tk.Button(self.root, text='Submit', command=self.submit)
        self.submitted_var = tk.StringVar()

        self.submitted_label = tk.Label(self.root, text="Submitted name: ")
        self.submitted_entry = tk.Label(self.root, textvariable=self.submitted_var)

    def setup(self):
        self.submitted_label.grid(row=0, column=0)
        self.submitted_entry.grid(row=0, column=1)
        self.name_label.grid(row=1, column=0)
        self.name_entry.grid(row=1, column=1)
        self.sub_btn.grid(row=2, column=1)

    def submit(self):
        self.submitted_var.set(self.name_entry.get())

    def run(self):
        self.setup()
        self.root.mainloop()

if __name__ == '__main__':
    Program().run()

Then you can use self.submitted_var.get() anywhere to get the last submitted value. Although, the more straight-forward way would be to link a StringVar directly to name_entry, but then it would change every time the Entry widget value changes, rendering the submit button useless.

How to return a value of a function given as a command in tkinter

2 Answers2