We can add an event listener function to a button by passing a call-back function to ```command``` parameter.  ```command=someFunction"

```python
import tkinter as tk

window = tk.Tk()

def printInfo:
    print("clicked....")

button = tk.Button(text="Login",
				command=printInfo)

button.pack()

window.mainloop()
```




Adding Event Listener to Button

We can use the destroy() method to quit a window.

```python
button = tk.Button(text="Quit",
					foreground="red",
					command=window.destroy)
button.pack()
```
We can pass ```command=window.destroy``` as a parameter to close a window with a button click


Closing the window using button click

A Button widget is similar to a Label widget, but it is clickable and can call a function. 
```python
button1 = tk.Button(text="Click Me",
				foreground="red")
button1.pack()
```


Georgia State University

GUI is the user interface where users can interact with the program using visual components like buttons, textbox, icons etc. 

Tkinter is a popular Python package and it is already included in Python 3. To start building GUI, all we need to do is import the package: ```import tkinter as tk```.

Python GUI Development with Tkinter

Real Python is a website that offers tutorials for different aspects of programming in Python: https://realpython.com

realpython

First import the tkinter library
```python
import tkinter as tk
```
We create an empty window by creating an instance of the Tk class
```python
window = tk.Tk();
```
Add a title to the window using the ```.title``` method
```python
window.title("1Cademy")
```
We can now add a label inside the window
```python
greeting = tk.Label(text="Hello, World")
greeting.pack()
```
Finally we add ```window.mainloop()``` to start an event listener . This method listens for any events like button clicks.
```python
window.mainloop()
```
After running these code a simple window will be created:


Creating a simple window with Python Tkinter library

Label : Display Text on the screen

Button : This contains a text and can perform certain action when clicked

Entry : This widget allows user to input a single line for Text

Text : This widget is similar to ```Entry``` but it accepts multiple Text input

Frame : This widget help to group together related widgets 

Basic Widgets in Tkinter

We can change different characteristics of labels like text color, background color, height, width etc. Text color is modified using foreground property.
```python
greeting = tk.Label(text="Hello, World",
				foreground="yellow",
				background="pink",
				width="10",
				height="5")
```


Modifying Labels

Creating Buttons with Tkinter

We use the ```.Entry()``` widget to get single line user input. For multiline, we can use ```Text``` widget.

We can add a Label before Entry widget to let the user know, what the input is for.

```python
label = tk.Label(text="Email:")
label.pack()

entry_email = tk.Entry()
entry_email.pack()

label = tk.Label(text="Password:")
label.pack()

entry_password = tk.Entry()
entry_password.pack()

button = tk.Button(text="Login",
					foreground="red")
button.pack()
```

Getting User Input using Entry Widget

Tkinter Documentation

We use ```pack()``` method to put widgets on the window. But it does not let us control where we want to place the widget. To position the widgets (label, entry, button etc) in the window, we can use the grid system to position widgets with more control. We assign rows and columns to the widgets. Rows and Columns start from 0. For example if we want a button at 1st row and 2nd column, we use row=1, column=2.  With the ```.grid()``` method, we dont need to use ```pack()```

```Syntax: .grid(row=, column=)

```python
import tkinter as tk

window = tk.Tk()
window.geometry("500x200")

#creating labels
label1 = tk.Label(text="Name:")
label2 = tk.Label(text="Age:")
label3 = tk.Label(text="School:")
label4 = tk.Label(text="Phone#")

button = tk.Button(text="Submit",
					foreground="red")
#creating entry
entry1 = tk.Entry()
entry2 = tk.Entry()
entry3 = tk.Entry()
entry4 = tk.Entry()

#positioning the widgets using the grid system
label1.grid(row=0, column=0)
entry1.grid(row=0, column=1)

label2.grid(row=1, column=0)
entry2.grid(row=1, column=1)

label3.grid(row=0, column=2)
entry3.grid(row=0, column=3)

label4.grid(row=1, column=2)
entry4.grid(row=1, column=3)

button.grid(row=2, column=2)

window.mainloop()
```


Grid System

messagebox is like a pop-up window that can be used to show user addition info or ask user a yes/no question.
First we need to import the module:
```python
from tkinter import messagebox
```
We then create a messagebox and call one of the built-in functions. We pass title text and message in the function.
```python
messagebox.showinfo("Title", "Enter message here")
```

messagebox in Tkinter

To use the checkbox we use the ```Checkbutton()``` class.

If we want to store the state of the checkbox (checked or unchecked), we can assign the state to a variable. 

Syntax:
```python
isChecked = IntVar()
check1 = Checkbutton(text='some text', variable='isChecked')
check1.pack()
```
By default, the state is 0 (unchecked) and 1 (checked). But we can change it using the ```onvalue``` and ```offvalue``` parameter:

```python
check1 = Checkbutton(text='some text', variable='isChecked', onvalue="Selected", offvalue="NotSelected")
```


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