An implementation of Lisp/Scheme-like cons in Python.

algebraic-data-types, cons, lisp-like, python, scheme-like
pip install cons==0.4.5


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Python cons

An implementation of cons in Python.

Usage and Design

The cons package attempts to emulate the semantics of Lisp/Scheme's cons as closely as possible while incorporating all the built-in Python sequence types:

>>> from cons import cons, car, cdr
>>> cons(1, [])

>>> cons(1, ())

>>> cons(1, [2, 3])
[1, 2, 3]

In general, cons is designed to work with types.

According to the cons package, None corresponds to the empty built-in list, as nil does in some Lisps:

>>> cons(1, None)

The cons package follows Scheme-like semantics for empty sequences:

>>> car([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ConsError: Not a cons pair

>>> cdr([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ConsError: Not a cons pair

By default, str types are not considered cons-pairs, although they are sequences:

>>> cons("a", "string")
ConsPair('a' 'a string')

This setting can be overridden and other types can be similarly excluded from consideration by registering classes with the abc-based classes MaybeCons and NonCons.


  • Built-in support for the standard Python ordered sequence types: i.e. list, tuple, Iterator, OrderedDict.
>>> from collections import OrderedDict
>>> cons(('a', 1), OrderedDict())
OrderedDict([('a', 1)])
  • Existing cons behavior can be changed and support for new collections can be added through the generic functions cons.core._car and cons.core._cdr.
  • Built-in support for unification.
>>> from unification import unify, reify, var
>>> unify([1, 2], cons(var('car'), var('cdr')), {})
{~car: 1, ~cdr: [2]}

>>> reify(cons(1, var('cdr')), {var('cdr'): [2, 3]})
[1, 2, 3]

>>> reify(cons(1, var('cdr')), {var('cdr'): None})


pip install cons


First obtain the project source:

git clone

Create a virtual environment and install the development dependencies:

$ pip install -r requirements.txt

Set up pre-commit hooks:

$ pre-commit install --install-hooks