Every Python developer has used None as a default but what happens when None is also a valid value? This talk explores sentinel values as a first-class concept in Python. We'll uncover why None is semantically overloaded, how built-in sentinels like NotImplemented and ... (Ellipsis) quietly power Python's object model, and what happens when you try to create your own. The central challenge: Python's type system has no good answer for sentinel values. We'll walk through why Optional, Literal, and custom classes all fall short, look at real-world workarounds including Pydantic's experimental MISSING, and discuss what PEP 661's deferral means for your code today. You'll leave with a clear mental model and practical guidance for handling absence in typed Python.