Creating our first Python GUI

Python is a very powerful programming language. It ships with the built-in tkinter module. In only a few lines of code (four, to be precise) we can build our first Python GUI.

Getting ready

To follow this recipe, a working Python development environment is a prerequisite. The IDLE GUI that ships with Python is enough to start. IDLE was built using tkinter!

Note

All the recipes in this book were developed using Python 3.4 on a Windows 7 64-bit OS. They have not been tested on any other configuration. As Python is a cross-platform language, the code from each recipe is expected to run everywhere.

If you are using a Mac, it does come built-in with Python, yet it might be missing some modules such as tkinter, which we will use throughout this book.

We are using Python 3 and the creator of Python intentionally chose not to make it backwards compatible with Python 2.

If you are using a Mac or Python 2, you might have to install Python 3 from www.python.org in order to successfully run the recipes in this book.

How to do it...

Here are the four lines of Python code required to create the resulting GUI:

import tkinter as tk     # 1
win = tk.Tk()            # 2
win.title("Python GUI")  # 3
win.mainloop()           # 4

Execute this code and admire the result:

How it works...

In line 1, we import the built-in tkinter module and alias it as tk to simplify our Python code. In line 2, we create an instance of the Tk class by calling its constructor (the parentheses appended to Tk turn the class into an instance). We are using the alias tk so we don't have to use the longer word tkinter. We are assigning the class instance to a variable named win (short for a window). As Python is a dynamically typed language, we did not have to declare this variable before assigning to it and we did not have to give it a specific type. Python infers the type from the assignment of this statement. Python is a strongly typed language, so every variable always has a type. We just don't have to specify its type beforehand like in other languages. This makes Python a very powerful and productive language to program in.

Note

A little note about classes and types:

In Python every variable always has a type. We cannot create a variable without assigning it a type. Yet, in Python, we do not have to declare the type beforehand, as we have to do in the C programming language.

Python is smart enough to infer the type. At the time of writing, C# also has this capability.

Using Python, we can create our own classes using the class keyword instead of the def keyword.

In order to assign the class to a variable, we first have to create an instance of our class. We create the instance and assign this instance to our variable.

class AClass(object):
    print('Hello from AClass')

classInstance = AClass()

Now the variable classInstance is of the type AClass.

If this sounds confusing, do not worry. We will cover OOP in the coming chapters.

In line 3, we use the instance variable of the class (win) to give our window a title via the title property. In line 4, we start the window's event loop by calling the mainloop method on the class instance win. Up to this point in our code, we created an instance and set one property but the GUI will not be displayed until we start the main event loop.

Note

An event loop is a mechanism that makes our GUI work. We can think of it as an endless loop where our GUI is waiting for events to be sent to it. A button click creates an event within our GUI or our GUI being resized also creates an event.

We can write all of our GUI code in advance and nothing will be displayed on the user's screen until we call this endless loop (win.mainloop() in the code shown above).

The event loop ends when the user clicks the red X button or a widget that we have programmed to end our GUI. When the event loop ends, our GUI also ends.

There's more...

This recipe used a minimum amount of Python code to create our first GUI program. However, throughout this book, we will use OOP when it makes sense.