In order to have a really solid foundation in Python, it's important to understand the Python  execution model. By this, we mean the rules defining precisely when function definitions and other important events occur during module import and execution. To help you develop this understanding, we'll focus on the def keyword since you're already familiar with it. Once you have an understanding of how def is processed by Python, you'll know most of what you need to know about Python's execution model.
What's important to understand is this: def isn't merely a declaration, it's a statement. What this means is that def is actually executed at runtime along with the rest of the top-level module-scope code. What def does is to bind the code in the function's body to the name following def. When modules are imported...
