In this chapter, we saw how to covert Python code into Cython. We also looked into some example Python code that involved NumPy arrays. We briefly explained the concept of boxing and unboxing in the Python language and how they affect the performance of code. We also explained how you can explicitly unlock the notorious GIL. To dig further deep in the Cython world, we recommend Learning Cython Programming, Philip Herron, Packt Publishing. In the next chapter, you will learn about the NumPy C API and how to use it.