Type Hints & mypy

Introduction

Python is dynamically typed but supports gradual typing via type hints (PEP 484). Tools like mypy and pyright check them statically.

Understanding the topic

Core concepts to understand:

• def f(x: int) -> str:
• list[int], dict[str, int] (3.9+).
• Optional[X] = X | None.
• Protocol for structural (duck) typing.

Syntax reference

Visual flow / code:

from typing import Protocol

def greet(name: str, times: int = 1) -> list[str]:
    return [f"hi {name}"] * times

# Optional + union
def find(id: int) -> User | None: ...

# Generics
def first[T](xs: list[T]) -> T: return xs[0]

# Protocol (structural typing)
class Comparable(Protocol):
    def __lt__(self, other) -> bool: ...

def smallest[T: Comparable](xs: list[T]) -> T:
    return min(xs)

Execution workflow

Real-world use

Modern Python codebases (FastAPI, Pydantic, Litestar) are entirely typed — type hints power runtime validation, IDE autocomplete, and docs.

Best practices

• Type all public functions.
• Run mypy/pyright in CI.
• Use Protocols over abstract base classes.

Hands-on exercise

Interview preparation — practice these questions:

• What is gradual typing?
• Protocol vs ABC?
• Does Python enforce hints at runtime?

Summary

In summary: Gradual typing = best of both. mypy/pyright + CI = safety.