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A set of utility functions for iterators, functions, and dictionaries.

See the PyToolz documentation at


New BSD. See License File.


toolz is on the Python Package Index (PyPI):

pip install toolz

Structure and Heritage

toolz is implemented in three parts:

itertoolz_, for operations on iterables. Examples: groupby, unique, interpose,

functoolz_, for higher-order functions. Examples: memoize, curry, compose,

dicttoolz_, for operations on dictionaries. Examples: assoc, update-in, merge.

These functions come from the legacy of functional languages for list processing. They interoperate well to accomplish common complex tasks.

Read our API Documentation for more details.


This builds a standard wordcount function from pieces within toolz:

>>> def stem(word):
...     """ Stem word to primitive form """
...     return word.lower().rstrip(",.!:;'-\"").lstrip("'\"")

>>> from toolz import compose, frequencies
>>> from toolz.curried import map
>>> wordcount = compose(frequencies, map(stem), str.split)

>>> sentence = "This cat jumped over this other cat!"
>>> wordcount(sentence)
{'this': 2, 'cat': 2, 'jumped': 1, 'over': 1, 'other': 1}


toolz supports Python 3.7+ with a common codebase. It is pure Python and requires no dependencies beyond the standard library.

It is, in short, a lightweight dependency.


The toolz project has been reimplemented in Cython. The cytoolz project is a drop-in replacement for the Pure Python implementation. See CyToolz GitHub Page for more details.

See Also

  • Underscore.js: A similar library for JavaScript
  • Enumerable: A similar library for Ruby
  • Clojure: A functional language whose standard library has several counterparts in toolz
  • itertools: The Python standard library for iterator tools
  • functools: The Python standard library for function tools

Contributions Welcome

toolz aims to be a repository for utility functions, particularly those that come from the functional programming and list processing traditions. We welcome contributions that fall within this scope.

We also try to keep the API small to keep toolz manageable. The ideal contribution is significantly different from existing functions and has precedent in a few other functional systems.

Please take a look at our issue page for contribution ideas.


See our mailing list. We're friendly.