Immutability

An immutable value is one that can't change after it's created. If you want a different value, you build a new one and leave the original alone. This sounds like a small detail, but it's one of the foundations of functional programming: pure functions stay pure when their inputs can't be mutated, shared data stays safe across threads when nobody can modify it, and reasoning about a program gets a lot easier when "the value of x" never depends on when you look at it. This chapter covers why immutability matters, which Python types are immutable out of the box, how to make your own classes immutable, what persistent data structures bring to the table, and what the trade-offs are in real Python code.