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*******************************************************************************
                                    absence
*******************************************************************************

.. image:: https://img.shields.io/pypi/v/absence
   :alt: Package Version
   :target: https://pypi.org/project/absence/

.. image:: https://img.shields.io/pypi/status/absence
   :alt: PyPI - Status
   :target: https://pypi.org/project/absence/

.. image:: https://github.com/emcd/python-absence/actions/workflows/tester.yaml/badge.svg?branch=master&event=push
   :alt: Tests Status
   :target: https://github.com/emcd/python-absence/actions/workflows/tester.yaml

.. image:: https://emcd.github.io/python-absence/coverage.svg
   :alt: Code Coverage Percentage
   :target: https://github.com/emcd/python-absence/actions/workflows/tester.yaml

.. image:: https://img.shields.io/github/license/emcd/python-absence
   :alt: Project License
   :target: https://github.com/emcd/python-absence/blob/master/LICENSE.txt

.. image:: https://img.shields.io/pypi/pyversions/absence
   :alt: Python Versions
   :target: https://pypi.org/project/absence/


🕳️ A Python library package which provides a **sentinel for absent values** - a
falsey, immutable singleton that represents the absence of a value in contexts
where ``None`` or ``False`` may be valid values.


Key Features ⭐
===============================================================================

* 1️⃣  **Absence Sentinel**: A falsey singleton which represents absence.
* 🏭 **Absence Factory**: Create custom absence sentinels for package-specific
  or arbitrary needs.
* 📦 **Absence Cell**: Immutable container wrapping absent values with
  conditional extraction, evaluation, and transformation API.
* 𝒇 **Predicate Functions**: Determine if a value is absent.
* 🔍 **Type Support**: Type alias for optional values which may be absent.
  (Similar to ``typing.Optional`` and its relation to ``None``.)
* 🌟 **Builtins Integration**: Can install singleton and predicate function
  into Python builtins.


Installation 📦
===============================================================================

::

    pip install absence


Examples 💡
===============================================================================

Use the ``absent`` sentinel to represent missing values:

>>> from dataclasses import dataclass
>>> from absence import absent, is_present, Absential
>>> @dataclass
... class User:
...     name: str | None
...     email: str | None
>>> def apply_partial_update(
...     user: User,
...     name: Absential[ str | None ] = absent,
...     email: Absential[ str | None ] = absent,
... ) -> User:
...     ''' Updates user fields if values provided.
...
...         Absent value means "don't change".
...         None value means "clear field".
...     '''
...     if is_present( name ): user.name = name
...     if is_present( email ): user.email = email
...     return user
>>> user = User( name = 'Alice', email = 'alice@example.com' )
>>> # Clear name but leave email unchanged
>>> updated = apply_partial_update( user, name = None )
>>> updated.name  # Cleared to None
>>> updated.email  # Unchanged
'alice@example.com'
>>> # Update both fields
>>> updated = apply_partial_update( user, name = 'Bob', email = 'bob@example.com' )
>>> updated.name
'Bob'
>>> updated.email
'bob@example.com'

Create package-specific absence sentinels:

>>> from absence import AbsenceFactory
>>> MISSING = AbsenceFactory( )
>>> bool( MISSING )
False

Wrap absent values with AbsenceCell for conditional chains:

>>> from absence import AbsenceCell, absent, Absential
>>> def adjust_columns( width: Absential[ int ] ) -> int:
...     ''' Returns adjusted width, or 0 if width is absent. '''
...     return AbsenceCell( width ).evaluate_or( lambda w: w - 4, 0 )
>>> adjust_columns( 80 )
76
>>> adjust_columns( absent )
0


Use Cases 🎯
===============================================================================

* 🔄 **Optional Arguments**: When ``None`` is a valid argument value but you
  need to detect absence.
* 🎯 **Sentinel Values**: When you need a unique, falsey object to represent
  missing or invalid states.
* 🧩 **Type Safety**: When you want explicit typing for values that may be
  absent.


Comparison with Alternatives 🤔
===============================================================================

+-------------------------+----------+---------+------------+------------+
| Alternative             | Truthy?  | Unique? | Picklable? | Scope      |
+=========================+==========+=========+============+============+
| ``object()``            | Yes      | Yes     | No         | Arbitrary  |
+-------------------------+----------+---------+------------+------------+
| PEP 661 Sentinels       | Optional | Yes     | Yes        | Per-Module |
+-------------------------+----------+---------+------------+------------+
| ``dataclasses.MISSING`` | Yes      | Yes     | No         | Global     |
+-------------------------+----------+---------+------------+------------+
| ``typing.NoDefault``    | Yes      | Yes     | Yes        | Global     |
+-------------------------+----------+---------+------------+------------+
| ``absence.absent``      | No       | Yes     | No         | Global     |
+-------------------------+----------+---------+------------+------------+

The ``absent`` sentinel combines falsey behavior with global uniqueness,
making it particularly suitable for representing missing values in contexts
where ``None`` might be a valid value. The companion ``AbsenceFactory``
allows creation of arbitrary absence sentinels, when needed, such as for
specific packages.

See `PEP 661 ("Sentinel Values") <https://peps.python.org/pep-0661/>`_,
`typing.NoDefault
<https://docs.python.org/3/library/typing.html#typing.NoDefault>`_, and
`dataclasses.MISSING
<https://docs.python.org/3/library/dataclasses.html#dataclasses.MISSING>`_ for
more details on alternatives.


Installation 📦
===============================================================================

Method: Install Python Package
-------------------------------------------------------------------------------

Install via `uv <https://github.com/astral-sh/uv/blob/main/README.md>`_ ``pip``
command:

::

    uv pip install absence

Or, install via ``pip``:

::

    pip install absence


Contribution 🤝
===============================================================================

Contribution to this project is welcome! However, it must follow the `code of
conduct
<https://emcd.github.io/python-project-common/stable/sphinx-html/common/conduct.html>`_
for the project.

Please file bug reports and feature requests in the `issue tracker
<https://github.com/emcd/python-absence/issues>`_ or submit `pull
requests <https://github.com/emcd/python-absence/pulls>`_ to
improve the source code or documentation.

For development guidance and standards, please see the `development guide
<https://emcd.github.io/python-absence/stable/sphinx-html/contribution.html#development>`_.


Additional Indicia
===============================================================================

.. image:: https://img.shields.io/github/last-commit/emcd/python-absence
   :alt: GitHub last commit
   :target: https://github.com/emcd/python-absence

.. image:: https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/copier-org/copier/master/img/badge/badge-grayscale-inverted-border-orange.json
   :alt: Copier
   :target: https://github.com/copier-org/copier

.. image:: https://img.shields.io/badge/%F0%9F%A5%9A-Hatch-4051b5.svg
   :alt: Hatch
   :target: https://github.com/pypa/hatch

.. image:: https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit
   :alt: pre-commit
   :target: https://github.com/pre-commit/pre-commit

.. image:: https://microsoft.github.io/pyright/img/pyright_badge.svg
   :alt: Pyright
   :target: https://microsoft.github.io/pyright

.. image:: https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json
   :alt: Ruff
   :target: https://github.com/astral-sh/ruff

.. image:: https://img.shields.io/pypi/implementation/absence
   :alt: PyPI - Implementation
   :target: https://pypi.org/project/absence/

.. image:: https://img.shields.io/pypi/wheel/absence
   :alt: PyPI - Wheel
   :target: https://pypi.org/project/absence/


Other Projects by This Author 🌟
===============================================================================


* `python-accretive <https://github.com/emcd/python-accretive>`_ (`accretive <https://pypi.org/project/accretive/>`_ on PyPI)

  🌌 A Python library package which provides **accretive data structures** - collections which can grow but never shrink.
* `python-classcore <https://github.com/emcd/python-classcore>`_ (`classcore <https://pypi.org/project/classcore/>`_ on PyPI)

  🏭 A Python library package which provides **foundational class factories and decorators** for providing classes with attributes immutability and concealment and other custom behaviors.
* `python-detextive <https://github.com/emcd/python-detextive>`_ (`detextive <https://pypi.org/project/detextive/>`_ on PyPI)

  🕵️ A Python library which provides consolidated text detection capabilities for reliable content analysis. Offers MIME type detection, character set detection, and line separator processing.
* `python-dynadoc <https://github.com/emcd/python-dynadoc>`_ (`dynadoc <https://pypi.org/project/dynadoc/>`_ on PyPI)

  📝 A Python library package which bridges the gap between **rich annotations** and **automatic documentation generation** with configurable renderers and support for reusable fragments.
* `python-falsifier <https://github.com/emcd/python-falsifier>`_ (`falsifier <https://pypi.org/project/falsifier/>`_ on PyPI)

  🎭 A very simple Python library package which provides a **base class for falsey objects** - objects that evaluate to ``False`` in boolean contexts.
* `python-frigid <https://github.com/emcd/python-frigid>`_ (`frigid <https://pypi.org/project/frigid/>`_ on PyPI)

  🔒 A Python library package which provides **immutable data structures** - collections which cannot be modified after creation.
* `python-icecream-truck <https://github.com/emcd/python-icecream-truck>`_ (`icecream-truck <https://pypi.org/project/icecream-truck/>`_ on PyPI)

  🍦 **Flavorful Debugging** - A Python library which enhances the powerful and well-known ``icecream`` package with flavored traces, configuration hierarchies, customized outputs, ready-made recipes, and more.
* `python-librovore <https://github.com/emcd/python-librovore>`_ (`librovore <https://pypi.org/project/librovore/>`_ on PyPI)

  🐲 **Documentation Search Engine** - An intelligent documentation search and extraction tool that provides both a command-line interface for humans and an MCP (Model Context Protocol) server for AI agents. Search across Sphinx and MkDocs sites with fuzzy matching, extract clean markdown content, and integrate seamlessly with AI development workflows.
* `python-mimeogram <https://github.com/emcd/python-mimeogram>`_ (`mimeogram <https://pypi.org/project/mimeogram/>`_ on PyPI)

  📨 A command-line tool for **exchanging collections of files with Large Language Models** - bundle multiple files into a single clipboard-ready document while preserving directory structure and metadata... good for code reviews, project sharing, and LLM interactions.
