In which popular Python libraries are type annotations used to specify data types?

Type annotations in Python are widely used in popular libraries to improve code readability and support static typing. Here are some examples:

• mypy — a static type checker that uses type annotations in code.
• pydantic — a library for data validation and model configuration using type annotations to describe data structures.
• FastAPI — a web framework that actively uses type annotations for automatic documentation generation and request validation.
• dataclasses (built-in module) — allows creating classes with type annotations for automatic method generation.

Example with pydantic:

from pydantic import BaseModel

class User(BaseModel):
    id: int
    name: str
    is_active: bool = True

Type annotations help tools and developers understand what data types are expected, reducing errors and improving IDE support.