examples.md

Sentinel Object — where it lives outside this repo

Cited, real implementations to study (or point an agent at) when designing or reviewing absence-handling code.

Python standard library

• dataclasses.MISSING. The stdlib's public missing-marker: it lets introspection distinguish "no default" from "default is None". docs.python.org/3/library/dataclasses.html
• inspect.Parameter.empty. The sentinel for "no default / no annotation" in signature introspection. docs.python.org/3/library/inspect.html
• unittest.mock.sentinel and mock.DEFAULT. Named sentinels offered as API — mint-a-marker as a service. docs.python.org/3/library/unittest.mock.html
• Two-argument iter(callable, sentinel). A sentinel baked into a builtin's signature: iteration stops when the sentinel appears. docs.python.org/3/library/functions.html#iter
• str.find's -1. The cautionary in-band sentinel value — legal integer, silently consumable by arithmetic.

Language evolution

• PEP 661 — Sentinel Values. The problem is real enough to have a PEP: naming, repr, and copy/pickle semantics for sentinels. peps.python.org/pep-0661
• The guide chapter — sentinel values vs the sentinel object vs the Null Object, with history. python-patterns.guide/python/sentinel-object

What to notice across all of them

The healthy examples are all out-of-band (a fresh object no domain can produce) and named (debuggable). The stdlib's one in-band survivor, str.find, is the pattern's standing warning label.