• All the recipes in this book were developed using Python 3.6 on a Windows 10 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 with built-in Python, yet it might be missing some modules such as tkinter, which we will use throughout this book.
• We are using Python 3.6, 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 3.6 from www.python.org in order to successfully run the recipes in this book.
• If you really wish to run the code in this book on Python 2.7, you will have to make some adjustments. For example, tkinter in Python 2.x has an uppercase T. The Python 2.7 print statement is a function in Python 3.6 and requires parentheses.
• While the EOL (End Of Life) for the Python 2.x branch has been extended to the year 2020, I would strongly recommend that you start using Python 3.6 and above.
• Why hold on to the past, unless you really have to?
  Here is a link to the Python Enhancement Proposal (PEP) 373 that refers to the EOL of Python 2: https://www.python.org/dev/peps/pep-0373/

A little note about classes and types:

• In Python, every variable always has a type. We cannot create a variable that does not have 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. C#, at the time of writing this book, 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, for example:
  class AClass(object):
      print('Hello from AClass') 
  class_instance = AClass()
  Now, the variable, class_instance, is of the AClass type.
  If this sounds confusing, do not worry. We will cover OOP in the coming chapters.

• 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 preceding code).
  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.