sentinel_object

id	python/sentinel_object
name	Sentinel Object
aliases	sentinel missing-marker
guide_url	https://python-patterns.guide/python/sentinel-object/
problem	Mark 'no value here' unambiguously when None itself is a legitimate value.
symptoms	None is a valid value distinguish missing from null default argument that could be None str.find returns -1
verdict	pythonic
caveats	A sentinel must be compared with `is`, never `==` — its identity is its meaning. Sentinel *values* like -1 (str.find) live inside the value's own type and eventually collide; a fresh object() cannot. Fowler's Null Object pattern — a do-nothing stand-in with real methods — is the neighboring cure when callers would otherwise be littered with None checks.
stdlib_sightings	dataclasses.MISSING iter(callable sentinel)

Sentinel Object

Mark "no value here" with an unforgeable object, so a legitimate None and a genuine absence stop colliding. Verdict: pythonic — one named marker per meaning, compared with is.

Where	What
pattern/	The importable code: Sentinel (named marker) and MISSING
docs/	Fundamentals · Implementation guide · External examples
examples/layered_config/	Mini-project: CLI ← file ← defaults config where None means "explicitly disabled", plus a NullNotifier
tests/	Behavioral tests for the pattern and the mini-project

uv run python -m patterns.python.sentinel_object.examples.layered_config.main