cons

library0.4.7pypypi✓ verified 28d ago

The `cons` library for Python provides an implementation of Lisp/Scheme-like cons cells, aiming to emulate their semantics while integrating with Python's built-in sequence types like lists and tuples. It facilitates functional programming patterns by allowing the construction and deconstruction of pairs. The library is actively maintained, with its current version being 0.4.7, and receives updates that include dependency bumps and Python version compatibility improvements.

serialization data

pip install cons

github ↗package ↗

INSTALL

IMPORT

SIG · CONS

cons

serializationpythonv0.4.7

Install

1.7s avg

Import

127ms

Disk

17MB

Pass rate

10/ 10

Env Coverage10 / 10

glibc

3.9–3.13

musl

3.9–3.13

Install & Compatibility

Where this runs

tested against v0.4.7 · pip install

no network on importno background threads

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

py 3.10–3.95 runs

installs and imports cleanly · install 0.0s · import 0.134s · 18.7MB

glibc

py 3.10–3.95 runs

installs and imports cleanly · install 1.7s · import 0.120s · 19MB

17MB installed

● package 17MB

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

cons

✓ from cons import cons

car

✓ from cons import car

cdr

✓ from cons import cdr

Demonstrates how to create cons cells with various Python sequence types and how to extract their `car` (head) and `cdr` (tail) components. It also shows `None` being treated as an empty list (nil) for consing.

from cons import cons, car, cdr # Create cons cells with different sequence types my_list_cons = cons(1, [2, 3]) my_tuple_cons = cons('a', ('b', 'c')) my_none_cons = cons(42, None) # None acts like 'nil' in some Lisps print(f"List cons: {my_list_cons}") print(f"Tuple cons: {my_tuple_cons}") print(f"None cons: {my_none_cons}") # Accessing the head (car) and tail (cdr) of a cons cell print(f"Car of list cons: {car(my_list_cons)}") # Output: 1 print(f"Cdr of list cons: {cdr(my_list_cons)}") # Output: [2, 3] # cons also works with dictionaries or other iterables from collections import OrderedDict my_ordereddict_cons = cons(('key', 'value'), OrderedDict()) print(f"OrderedDict cons: {my_ordereddict_cons}")

Debug

Known issues

breakingPython 3.7 support was deprecated in version 0.4.6. Users on Python 3.7 or older must upgrade their Python interpreter to at least 3.9 to use newer versions of `cons`.

fix

Upgrade Python environment to version 3.9 or higher.

affects: >=0.4.6

gotchaAttempting to call `car()` or `cdr()` on an empty Python sequence (e.g., `[]`, `()`) will raise a `ConsError`, as these are not considered valid `cons` pairs within the library's Scheme-like semantics.

fix

Always ensure the argument to `car()` or `cdr()` is a valid cons pair or a sequence that has been created with `cons()` and is not empty. Check for emptiness before deconstructing.

affects: All

gotchaBy default, `str` types are not treated as `cons`-pairs for `car`/`cdr` operations, even though they are Python sequences. Calling `cons("a", "string")` will produce a `ConsPair` object, but `car("string")` would raise `ConsError`.

fix

If `str` type consing and deconstruction is desired, the library's behavior for considering types as `Cons`-pairs can be overridden by registering classes with `MaybeCons` and `NonCons` from `cons.core`.

affects: All

Errors

Common errors & fixes

ModuleNotFoundError: No module named 'cons'

The 'cons' library or its specific modules have not been installed or are not accessible in the current Python environment.

fix

Install the library using pip: `pip install cons`

AttributeError: module 'cons' has no attribute 'cons'

The user attempted to call `cons.cons()` (or `cons.car()`, `cons.cdr()`) after using `import cons`, but the functions `cons`, `car`, and `cdr` are intended to be imported directly from the package, not accessed as attributes of the `cons` module itself.

fix

Import the functions directly: `from cons import cons, car, cdr`

TypeError: 'int' object is not subscriptable

The `car` or `cdr` function was called with an argument that is not a sequence type (like a list or tuple) or a `cons` object, and the function internally attempted to access it using subscripting (e.g., `obj[0]`).

fix

Ensure that `car` and `cdr` are called with valid `cons` objects or Python sequences that represent a cons structure: `car(cons(1, []))` or `car([1, 2, 3])`.

TypeError: cons() missing 1 required positional argument: 'cdr'

The `cons` function was called with only one argument, but it requires two positional arguments: `car` (the first element) and `cdr` (the rest of the list/sequence or another cons cell).

fix

Provide both arguments to the `cons` function: `my_cons_cell = cons(1, [])` or `my_cons_cell = cons('a', ('b',))`

Upgrade

Version history

0.4.7latest on PyPI · released Jul 11, 2025

Audit

Dependencies

pythonrequiredRequires Python 3.9 or newer.

Agent activity

10 hits · last 30 days

node

OpenAI (training)

Resources

githubhttps://github.com/pythological/python-cons ↗

packagehttps://pypi.org/project/cons/ ↗