typevar
Domain: Validation Typing
🎯 Motivation
TypeVar is a type variable used as a placeholder in Generic definitions. It lets you express relationships like “the input and output must be the same type” without naming a specific one.
📋 Concrete Example
T = TypeVar("T") then def first(items: list[T]) -> T tells mypy that first([1,2,3]) returns int and first(["a"]) returns str, preserving the element type.
🔗 Analogy
TypeVar is like a blank named slot in a template. You fill it with a concrete type at each use, and the template guarantees consistency across all references to that slot.