The topic of this book is Reinforcement Learning—which is a subfield of Machine Learning—focusing on the general and challenging problem of learning optimal behavior in complex environment. The learning process is driven only by reward value and observations obtained from the environment. This model is very general and can be applied to many practical situations from playing games to optimizing complex manufacture processes.

Due to flexibility and generality, the field of Reinforcement Learning is developing very quickly and attracts lots of attention both from researchers trying to improve existing or create new methods, as well as from practitioners interested in solving their problems in the most efficient way.

This book was written as an attempt to fill the obvious lack of practical and structured information about Reinforcement Learning methods and approaches. On one hand, there are lots of research activity all around the world, new research papers are being published almost every day, and a large portion of Deep Learning conferences such as NIPS or ICLR is dedicated to RL methods. There are several large research groups focusing on RL methods application in Robotics, Medicine, multi-agent systems, and others. The information about the recent research is widely available, but is too specialized and abstract to be understandable without serious efforts. Even worse is the situation with the practical aspect of RL application, as it is not always obvious how to make a step from the abstract method described in the mathematical-heavy form in a research paper to a working implementation solving actual problem. This makes it hard for somebody interested in the field to get an intuitive understanding of methods and ideas behind papers and conference talks. There are some very good blog posts about various RL aspects illustrated with working examples, but the limited format of a blog post allows the author to describe only one or two methods without building a complete structured picture and showing how different methods are related to each other. This book is my attempt to address this issue.

Another aspect of the book is its orientation to practice. Every method is implemented for various environments, from very trivial to quite complex. I've tried to make examples clean and easy to understand, which was made possible by the expressiveness and power of PyTorch. On the other hand, complexity and requirements of examples are oriented to RL hobbyists without access to very large computational resources, such as clusters of GPUs or very powerful workstations. This, I believe, will make the fun-filled and exciting RL domain accessible for a much wider audience than just research groups or large AI companies. However, it is still Deep Reinforcement Learning, so, having access to a GPU is highly recommended. Approximately, half of the examples in the book will benefit from running them on GPU. In addition to traditional medium-sized examples of environments used in RL, such as Atari games or continuous control problems, the book contains three chapters (8, 12, and 13) that contain larger projects, illustrating how RL methods could be applied to more complicated environments and tasks. These examples are still not full-sized real-life projects (otherwise they'll occupy a separate book on their own), but just larger problems illustrating how the RL paradigm can be applied to domains beyond well-established benchmarks.

Another thing to note about examples in the first three parts of the book is that I've tried to make examples self-contained and the source code was shown in full. Sometimes this led to repetition of code pieces (for example, training loop is very similar in most of the methods), but I believe that giving you the freedom to jump directly into the method you want to learn is more important than avoiding few repetitions. All examples in the book is available on Github: https://github.com/PacktPublishing/Deep-Reinforcement-Learning-Hands-On, and you're welcome to fork them, experiment, and contribute.