Chapter 3: Your First Application

Start with a vertical slice: route, model, service, test. Do not start with a grand architecture.

from typing import Annotated

from fastapi import FastAPI, HTTPException, Path
from pydantic import BaseModel, Field

app = FastAPI(title="RecipeVault")


class RecipeSummary(BaseModel):
    id: int
    title: str = Field(min_length=1, max_length=120)
    servings: int = Field(ge=1, le=50)


RECIPES = {
    1: RecipeSummary(id=1, title="Sourdough Pancakes", servings=4),
}


@app.get("/api/recipes/{recipe_id}", response_model=RecipeSummary)
def get_recipe(
    recipe_id: Annotated[int, Path(ge=1)],
) -> RecipeSummary:
    recipe = RECIPES.get(recipe_id)
    if recipe is None:
        raise HTTPException(status_code=404, detail="Recipe not found")
    return recipe

Key ideas:

• Type hints are runtime inputs for FastAPI.
• Pydantic models describe validated data contracts.
• HTTPException is the normal way to stop request handling with a specific HTTP result.
• response_model shapes output and OpenAPI metadata.

In .NET, you might separate DTOs, entities, services, and endpoints immediately. In Python, begin simple, then extract when there is real pressure. Python projects become hard to maintain when they are either too clever or too flat for too long.