What are generics and TypeVar in Python?

Question presented to candidate: "Say I write def first(items: list) -> object. A caller passes a list of ints and gets back something the type checker only knows is an object. How do generics and TypeVar fix that, and what does a generic class look like?"

What a strong answer should cover:

• A type variable (T = TypeVar("T")) is a placeholder the checker solves at each call, so first(list[int]) returns int: it links input and output types, which Any cannot do
• Generic classes subclass Generic[T] (or use class Stack[T]: on 3.12+); Stack[int] is checked as a stack of ints
• Bounds (TypeVar("N", bound=float)) allow any subtype and keep it; constraints (TypeVar("S", str, bytes)) allow exactly one of a fixed list
• Variance: list is invariant (a list[int] is not a list[float]), read-only types like Sequence are covariant; a mutable write is why
• Type erasure at runtime: Stack[int]() is a plain Stack, the parameter is never checked, and isinstance(x, Stack[int]) raises TypeError

Code / implementation expected: Optional — a generic function such as first or a small generic container class, ideally with what a type checker infers for it.