Fluent Python, 2nd Edition

Chapter 7. Functions as First-Class Objects

I have never considered Python to be heavily influenced by functional languages, no matter what people say or think. I was much more familiar with imperative languages such as C and Algol 68 and although I had made functions first-class objects, I didn’t view Python as a functional programming language.

Guido van Rossum, Python BDFL¹

Functions in Python are first-class objects. Programming language researchers define a “first-class object” as a program entity that can be:

• Created at runtime

• Assigned to a variable or element in a data structure

• Passed as an argument to a function

• Returned as the result of a function

Integers, strings, and dictionaries are other examples of first-class objects in Python—nothing fancy here. Having functions as first-class objects is an essential feature of functional languages, such as Clojure, Elixir, and Haskell. However, first-class functions are so useful that they’ve been adopted by popular languages like JavaScript, Go, and Java (since JDK 8), none of which claim to be “functional languages.”

This chapter and most of Part III explore the practical applications of treating functions as objects.

Tip

The term “first-class functions” is widely used as shorthand for “functions as first-class objects.” It’s not ideal because it implies an “elite” among functions. In Python, all functions are first-class.

What’s New in This Chapter

The section “The Nine Flavors of Callable Objects” was titled ...