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What Is Functional Programming?

We’d better start with the hardest question: "What is functional programming (FP), anyway?"

One answer would be to say that functional programming is what you do when you program in languages like Lisp, Scheme, Clojure, Scala, Haskell, ML, OCAML, Erlang, or a few others. That is a safe answer, but not one that clarifies very much. Unfortunately, it is hard to get a consistent opinion on just what functional programming is, even from functional programmers themselves. A story about elephants and blind men seems apropos here. It is also safe to contrast functional programming with "imperative programming" (what you do in languages like C, Pascal, C++, Java, Perl, Awk, TCL, and most others, at least for the most part). Functional programming is also not object-oriented programming (OOP), although some languages are both. And it is not Logic Programming (e.g., Prolog), but again some languages are multiparadigm.

Personally, I would roughly characterize functional programming as having at least several of the following characteristics. Languages that get called functional make these things easy, and make other things either hard or impossible:

  • Functions are first class (objects). That is, everything you can do with "data" can be done with functions themselves (such as passing a function to another function).

  • Recursion is used as a primary control structure. In some languages, no other "loop" construct exists.

  • There is a focus on list processing (for example, it is the source of the name Lisp). Lists are often used with recursion on sublists as a substitute for loops.

  • "Pure" functional languages eschew side effects. This excludes the almost ubiquitous pattern in imperative languages of assigning first one, then another value to the same variable to track the program state.

  • Functional programming either discourages or outright disallows statements, and instead works with the evaluation of expressions (in other words, functions plus arguments). In the pure case, one program is one expression (plus supporting definitions).

  • Functional programming worries about what is to be computed rather than how it is to be computed.

  • Much functional programming utilizes "higher order" functions (in other words, functions that operate on functions that operate on functions).

Advocates of functional programming argue that all these characteristics make for more rapidly developed, shorter, and less bug-prone code. Moreover, high theorists of computer science, logic, and math find it a lot easier to prove formal properties of functional languages and programs than of imperative languages and programs. One crucial concept in functional programming is that of a "pure function"—one that always returns the same result given the same arguments—which is more closely akin to the meaning of "function" in mathematics than that in imperative programming.

Python is most definitely not a “pure functional programming language”; side effects are widespread in most Python programs. That is, variables are frequently rebound, mutable data collections often change contents, and I/O is freely interleaved with computation. It is also not even a “functional programming language” more generally. However, Python is a multiparadigm language that makes functional programming easy to do when desired, and easy to mix with other programming styles.

Beyond the Standard Library

While they will not be discussed withing the limited space of this report, a large number of useful third-party Python libraries for functional programming are available. The one exception here is that I will discuss Matthew Rocklin’s multipledispatch as the best current implementation of the concept it implements.

Most third-party libraries around functional programming are collections of higher-order functions, and sometimes enhancements to the tools for working lazily with iterators contained in itertools. Some notable examples include the following, but this list should not be taken as exhaustive:

  • pyrsistent contains a number of immutable collections. All methods on a data structure that would normally mutate it instead return a new copy of the structure containing the requested updates. The original structure is left untouched.

  • toolz provides a set of utility functions for iterators, functions, and dictionaries. These functions interoperate well and form the building blocks of common data analytic operations. They extend the standard libraries itertools and functools and borrow heavily from the standard libraries of contemporary functional languages.

  • hypothesis is a library for creating unit tests for finding edge cases in your code you wouldn’t have thought to look for. It works by generating random data matching your specification and checking that your guarantee still holds in that case. This is often called property-based testing, and was popularized by the Haskell library QuickCheck.

  • more_itertools tries to collect useful compositions of iterators that neither itertools nor the recipes included in its docs address. These compositions are deceptively tricky to get right and this well-crafted library helps users avoid pitfalls of rolling them themselves.


There are a large number of other papers, articles, and books written about functional programming, in Python and otherwise. The Python standard documentation itself contains an excellent introduction called "Functional Programming HOWTO," by Andrew Kuchling, that discusses some of the motivation for functional programming styles, as well as particular capabilities in Python.

Mentioned in Kuchling’s introduction are several very old public domain articles this author wrote in the 2000s, on which portions of this report are based. These include:

  • The first chapter of my book Text Processing in Python, which discusses functional programming for text processing, in the section titled “Utilizing Higher-Order Functions in Text Processing.”

I also wrote several articles, mentioned by Kuchling, for IBM’s developerWorks site that discussed using functional programming in an early version of Python 2.x:

Not mentioned by Kuchling, and also for an older version of Python, I discussed multiple dispatch in another article for the same column. The implementation I created there has no advantages over the more recent multipledispatch library, but it provides a longer conceptual explanation than this report can:

A Stylistic Note

As in most programming texts, a fixed font will be used both for inline and block samples of code, including simple command or function names. Within code blocks, a notional segment of pseudo-code is indicated with a word surrounded by angle brackets (i.e., not valid Python), such as <code-block>. In other cases, syntactically valid but undefined functions are used with descriptive names, such as get_the_data().

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