Expected Answer (Python 3.14)

Type hints are optional annotations that document an interface and enable static analysis. Python normally stores annotations but does not use them to reject an invalid argument at runtime. A type checker can flag a mismatch before execution, while a validation library is needed when untrusted runtime data must be checked.

Use precise built-in generic types and unions to communicate the contract. A Protocol describes the methods and attributes a value must provide, allowing structural typing without forcing a shared base class.

from typing import Protocol

class HasName(Protocol):
    name: str

def greet(person: HasName, prefix: str = "Hello") -> str:
    return f"{prefix}, {person.name}!"

class User:
    name = "Ada"

assert greet(User()) == "Hello, Ada!"

Why It Matters

Good annotations turn implicit expectations into checkable contracts. They improve editor support, refactoring confidence, and collaboration while retaining Python’s runtime flexibility.

Common Mistakes

• Expecting annotations to validate runtime input automatically: Static type checkers do not run as part of normal execution.
• Using Any to silence every error: It disables useful checking and lets unsound assumptions spread.
• Using object as if it supported arbitrary operations: Every value is an object, but only operations defined on object are type-safe without narrowing.

Follow-up Questions

• What does T | None mean? (Answer: The value may be a T or None; it is the modern spelling of Optional[T].)
• Why use Protocol rather than an abstract base class? (Answer: It supports structural compatibility without requiring explicit inheritance.)