In this chapter, we'll cover how to use corpus readers and create custom corpora. If you want to train your own model, such as a part-of-speech tagger or text classifier, you will need to create a custom corpus to train on. Model training is covered in the subsequent chapters.

Now you'll learn how to use the existing corpus data that comes with NLTK. This information is essential for future chapters when we'll need to access the corpora as training data. You've already accessed the WordNet corpus in Chapter 1, Tokenizing Text and WordNet Basics. This chapter will introduce you to many more corpora.

We'll also cover creating custom corpus readers, which can be used when your corpus is not in a file format that NLTK already recognizes, or if your corpus is not located in files at all, but instead is located in a database such as MongoDB. It is essential to be familiar with tokenization, which was covered in Chapter 1, Tokenizing Text and WordNet Basics.