Type Hints — They Do Nothing at Runtime

def add(a: int, b: int) -> int:
    return a + b

add('hello', 'world')  # No error! Returns 'helloworld'

Python runtime completely ignores type hints. Stored in __annotations__ dict but never checked. A design decision — maintaining dynamic typing flexibility while adding tooling support.

So What Are They For?

1. mypy/pyright — catches type mismatches before execution
2. IDE autocomplete — VS Code shows methods for the type on .
3. Documentation — readers immediately see arg/return types

How Pydantic Does Runtime Type Checking

Pydantic reads __annotations__, generates per-field validators, and validates on __init__. V2 runs validation logic in Rust (pydantic-core).

Python type hints do nothing by themselves, but Pydantic reads that "does-nothing" metadata to build runtime validation.