Type Validation

Annotated

• Syntax:

  from typing import Annotated
  Annotated[type, metadata]
  

• What it does: Adds extra information (metadata) to a type without changing the type itself. Python ignores the metadata, but frameworks (like FastAPI, Pydantic) use it for:

  • validation
  • documentation
  • constraints

• Example:

  age: Annotated[int, "must be > 0"]
  

  → still an int, but with extra info attached

• When to use: When libraries need more info about your types.

from pydantic import BaseModel

class User(BaseModel):
    id: int
    name: str

user = User(id=1, name="Alice")

# Convert to dict
user_dict = user.model_dump()
print(user_dict)  # {'id': 1, 'name': 'Alice'}