1.1 The functional style of programming

We’ll define functional programming through a series of examples. The distinguishing feature between these examples is the concept of state, specifically the state of the computation.

Python’s strong imperative traits mean that the state of a computation is defined by the values of the variables in the various namespaces. Some kinds of statements make a well-defined change to the state by adding, changing, or removing a variable. We call this imperative because specific kinds of statements change the state.

In Python, the assignment statement is the primary way to change the state. Python has other statements, such as global or nonlocal, which modify the rules for variables in a particular namespace. Statements such as def, class, and import change the processing context. The bulk of the remaining statements provide ways to choose which assignment statements get executed. The focus of all these various statement types, however, is on changing the state of the variables.

In a functional language, we replace the state—the changing values of variables—with a simpler notion of evaluating functions. Each function evaluation creates a new object or objects from existing objects. Since a functional program is a composition of functions, we can design lower-level functions that are easy to understand, and then create compositions of functions that can also be easier to visualize than a complex sequence of statements.

Function evaluation more closely parallels mathematical formalisms. Because of this, we can often use simple algebra to design an algorithm that clearly handles the edge cases and boundary conditions. This makes us more confident that the functions work. It also makes it easy to locate test cases for formal unit testing.

It’s important to note that functional programs tend to be relatively succinct, expressive, and efficient compared to imperative (object-oriented or procedural) programs. The benefit isn’t automatic; it requires careful design. This design effort for functional programming is often smaller than for procedural programming. Some developers experienced in imperative and object-oriented styles may find it a challenge to shift their focus from stateful designs to functional designs.