Chapter 7: Request Binding and Validation

ASP.NET Core binds route values, query strings, headers, forms, and bodies. FastAPI does the same using function parameters, Annotated, and Pydantic models.

from typing import Annotated

from fastapi import Header, Query
from pydantic import BaseModel, Field


class RecipeSearch(BaseModel):
    q: str | None = Field(default=None, max_length=100)
    page: int = Field(default=1, ge=1)
    page_size: int = Field(default=20, ge=1, le=100)


@router.get("")
def search_recipes(
    filters: Annotated[RecipeSearch, Query()],
    accept_language: Annotated[str | None, Header()] = None,
):
    ...

Python validation practices:

• Prefer Pydantic constraints over manual if checks for shape validation.
• Keep business rules in services or domain functions.
• Use field_validator and model_validator for validation that belongs to the data contract.
• Return consistent error shapes. FastAPI gives a default 422 validation response; decide whether to keep it or wrap it.

ASP.NET Core developers often ask where validation attributes went. In Python, validation usually lives in Pydantic model fields and validators.