simplibs-sentinels

Shared sentinel values for the simplibs ecosystem — precise tools for distinguishing different states of absence and intent.

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Contents

• Installation
• Why sentinels?
• Overview
• UNSET
• MISSING
• DEFAULT
• EMPTY
• Type annotations
• About the simplibs ecosystem

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Installation

pip install simplibs-sentinels

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Why sentinels?

Python has no built-in way to distinguish between "parameter was not provided", "value is intentionally absent", and "value is None" — yet these are three different situations requiring different logic.

# Problem — None can mean anything
def connect(host: str, timeout: int | None = None):
    if timeout is None:
        # was timeout omitted? or intentionally passed as None?
        ...

# Solution — each state has its own sentinel
def connect(host: str, timeout: int | UnsetType = UNSET):
    if timeout is UNSET:
        timeout = get_default_timeout()  # not provided → use default
    elif timeout is None:
        timeout = 0                      # None passed intentionally → no timeout

All sentinels are singletons and falsy — always compare using is.

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Overview

Sentinel	Semantics	Typical context
UNSET	function parameter was not provided	default parameter values
MISSING	value was expected but is absent from input data	input data validation
DEFAULT	explicit request for default behaviour	overriding dynamic defaults
EMPTY	intentional emptiness, distinct from None	clearing or resetting values

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UNSET

Distinguishes a parameter that was not provided from an intentionally passed None.

from simplibs.sentinels import UNSET, UnsetType

def connect(host: str, timeout: int | UnsetType = UNSET):
    if timeout is UNSET:
        timeout = get_default_timeout()
    elif timeout is None:
        timeout = 0

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MISSING

Signals that a value was expected but is entirely absent from the input data. Useful when validating dictionaries, forms, or API payloads.

from simplibs.sentinels import MISSING, MissingType

def validate(data: dict, key: str):
    value = data.get(key, MISSING)
    if value is MISSING:
        raise ValueError(f"Required key '{key}' is missing.")
    elif value is None:
        ...  # key exists, value is None — different logic applies

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DEFAULT

Expresses the explicit intent "I want the default behaviour" — even when the default value is not static but decided at runtime.

from simplibs.sentinels import DEFAULT, DefaultType

def render(color: str | DefaultType = DEFAULT):
    if color is DEFAULT:
        color = theme.primary_color

Difference from UNSET: UNSET means "the user provided nothing", DEFAULT means "the user explicitly wants the default value".

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EMPTY

Intentional emptiness — a signal that a value should be explicitly cleared, without needing to pass an empty collection of a specific type.

from simplibs.sentinels import UNSET, EMPTY, EmptyType

def set_tags(tags: list | EmptyType | UnsetType = UNSET):
    if tags is UNSET:
        pass              # not provided → change nothing
    elif tags is EMPTY:
        self.tags = []    # intentionally clear all tags
    else:
        self.tags = tags

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Type annotations

Use the sentinel types directly in type annotations:

from simplibs.sentinels import UNSET, UnsetType

def process(value: str | UnsetType = UNSET) -> None:
    ...

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About the simplibs ecosystem

simplibs-sentinels is part of the simplibs ecosystem — a collection of small, self-contained Python libraries. Each one solves exactly one thing — but all of them share a common philosophy:

Dyslexia-friendly — minimise mental load. Atomise code into self-contained units, name files after the logic they contain, write explanations that describe why — not just what.

Programmer's zen — nothing should be missing and nothing should be superfluous. The journey is the destination: code should be fully understood; better to go slowly and correctly than quickly and with mistakes. The crystallisation approach — not perfection on the first try, but gradual refinement towards it.

Defensive style — anticipate all possible failure modes so that only safe paths remain. Never raise unexpected errors; degrade gracefully.

Minimalism — find the path to the goal in as few steps as possible, but leave nothing out. Each file has one responsibility.

Code as craft — code should be pleasant to look at and evoke a sense of harmony. Treat code as a small work of art — like a carpenter carving a sculpture. Optimise for the user: everything should make sense without having to study the documentation at length.

These are aspirations — a sense of direction. And that is exactly what the note about the journey becoming the destination is all about. 🙂

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The library is covered by tests across all modules. Tests are part of the repository and serve as living documentation of the expected behaviour.