While Python was originally intended as an imperative language [Guido], it contains all elements necessary to support a rich set of features from the functional paradigm. In particular its core data structures, lazy iterators, and functions as first class objects can be combined to implement a common standard library of functions shared among many functional languages.

This was first recognized and supported through the standard libraries itertools and functools which contain functions like permutations, chain and partial to complement the standard map, filter, reduce already found in the core language. While these libraries contain substantial functionality they do not achieve the same level of adoption found in similar projects in other languages. This may be because they are incomplete and lack a number of commonly related functions like compose and groupby which often complement these core operations.

A completion of this set of functions was first attempted in the projects itertoolz and functoolz (note the z). These libraries contained several functions that were absent in the standard itertools/functools libraries. The itertoolz/functoolz libraries were eventually merged into the monolithic toolz project described here.

Most contemporary functional languages (Haskell, Scala, Clojure, …) contain some variation of the functions found in toolz. The toolz project generally adheres closely to the API found in the Clojure standard library (see cheatsheet) and where disagreements occur that API usually dominates. The toolz API is also strongly affected by the principles of the Python language itself, and often makes deviations in order to be more approachable to that community.

The development of a functional standard library within a popular imperative language is not unique. Similar projects have arisen in other imperative-by-design languages that contain the necessary elements to support a functional standard library. Underscore.js in JavaScript has attained notable popularity in the web community. LINQ in C# follows a similar philosophy but mimics declarative database languages rather than functional ones. Enumerable is is the closest project in Ruby. Other excellent projects also exist within the Python ecosystem, most notably Fn.py and Funcy.