Peter Otten __peter__ at
Sat Jan 28 08:58:54 EST 2017

hany.amin.mishriky at wrote:

> hay , i am new in the coding world,i would like to understand how a python
> program is communicating with GUI, for example, if i have a code that
> require the user to enter a value ,then this code will do some
> calculations and return a value to the user, how to do that?

A command line script adding two numbers will look like this:

x = input_int("Enter x:")
y = input_int("Enter y:")
print("x + y =", x + y)

This script will not continue to ask for y until the user has entered the 
value for x

In a GUI you have to provide widgets that allow the user to interact with 
the GUI . Let's assume make_widget() creates an entry widget and shows it in 
a window. Then we can create the part of the GUI where the user enters the x 
and y value with:

entry_x = make_widget()
entry_y = make_widget()

Now we need one more widget to show the result:

entry_sum = make_widget()

We also need a function that recalculates the contents of entry_sum whenever 
the user changes x or y:

def recalculate_sum():
    x = int(entry_x.get_value())
    y = int(entry_y.get_value())

But when should this function run? We have to connect it to the widgets on 
whose changes it should react. Such a function is called "callback":

# assume that every time the contents of entry_x/y change it calls
# self.on_change()
entry_x.on_change = recalculate_sum
entry_y.on_change = recalculate_sum

Finally you enter an infinite loop that waits for and handles GUI events, 
the most interesting being user activities like typing a digit into one of 
your widgets.


There a many GUIs, but they all tend to work in a way similar to the one 
sketched above. Here's a translation of the above pseudo-code to tkinter:

import tkinter as tk

def make_entry(caption, row):
    """Create a Label and an Entry.
    Return the StringVar associated with the Entry.
    # the text displayed to the left of the Entry
    label = tk.Label(root, text=caption)
    label.grid(row=row, column=0)

    var = tk.StringVar() # holds the text entered into the Entry
    entry = tk.Entry(root, textvariable=var)
    entry.grid(row=row, column=1, sticky=tk.EW)
    return var

def calculate_sum(*args):
    """Callback invoked on changes of x or y

    The GUI passes some arguments (*args) that we are not interested in.
        x = int(var_x.get())
        y = int(var_y.get())
        sigma = x + y
    except ValueError as err:
        # if x or y aren't valid integers
        # show an error message instead of the sum
        sigma = err

# Create the main window
root = tk.Tk()

# Have the second column take all extra space
root.columnconfigure(1, weight=1)

var_x = make_entry("x", 0)
var_y = make_entry("y", 1)
var_sum = make_entry("x + y =", 2)

# Tell the GUI to invoke calculate_sum() after every change of x or y
var_x.trace("w", calculate_sum)
var_y.trace("w", calculate_sum)


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