We’ve completed hundreds of machine learning projects using dozens of different packages, and many different programming languages. At fast.ai, we have written courses using most of the main deep learning and machine learning packages used today. After PyTorch came out in 2017 we spent over a thousand hours testing it before deciding that we would use it for future courses, software development, and research. Since that time PyTorch has become the world’s fastest-growing deep learning library and is already used for most research papers at top conferences. This is generally a leading indicator of usage in industry, because these are the papers that end up getting used in products and services commercially. We have found that PyTorch is the most flexible and expressive library for deep learning. It does not trade off speed for simplicity, but provides both.

However, it doesn’t really matter what software you learn, because it takes only a few days to learn to switch from one library to another. What really matters is learning the deep learning foundations and techniques properly. Our focus will be on using code that clearly expresses the concepts that you need to learn. Where we are teaching high-level concepts, we will use high-level fastai code. Where we are teaching low-level concepts, we will use low-level PyTorch, or even pure Python code.

By the end of the book, you’ll understand nearly all the code that’s inside fastai (and much of PyTorch too), because in each chapter we’ll be digging a level deeper to show you exactly what’s going on as we build and train our models. This means that you’ll have learned the most important best practices used in modern deep learning—not just how to use them, but how they really work and are implemented. If you want to use those approaches in another framework, you’ll have the knowledge you need to do so if needed.

Let’s see it in practice and train our first model.