Cons Python Library

0.4.7 · active · verified Sat Apr 11

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.

Warnings

breaking Python 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.
gotcha Attempting 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.
gotcha By 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`.

Install

pip install cons Install latest version

Imports

cons
```
from cons import cons
```
car
```
from cons import car
```
cdr
```
from cons import cdr
```

Quickstart

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}")

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