FastAPI · Foundations

App instance

Create the app object and set the title and version shown in its docs.

core

Overview

Everything starts with app = FastAPI(). That one object is the ASGI application the server runs, the registry that route decorators attach to, and the home of app-wide configuration: docs metadata, middleware, exception handlers, lifespan, and global dependencies.

Key concepts

main:app: How the server finds your application: module main, variable app. Rename either and the run command changes with it.
Metadata: title, version, description and tags appear in the docs and the OpenAPI schema. They describe the API; they do not change how it behaves.
docs_url / redoc_url: Set to None to switch the interactive docs off - common for internal APIs in production.
Return values: Return a dict, list, Pydantic model or dataclass and FastAPI converts it to JSON. Return a Response object to take full control of the output.

Best practices

Create the app in one place - main.py or a create_app() factory - so tests can build a fresh instance.
Keep version in step with your releases so clients can tell what they are talking to.
Hide or protect the docs for internal-only APIs rather than publishing the full schema.

Creating the app

FastAPI() - all constructor params

Core

from fastapi import FastAPI

app = FastAPI(
    title="My API",
    description="API description shown in docs",
    version="1.0.0",
    docs_url="/docs",       # set None to disable Swagger
    redoc_url="/redoc",     # set None to disable ReDoc
    openapi_url="/openapi.json",
    terms_of_service="https://example.com/terms",
    contact={"name": "Dev", "email": "dev@rte.dev"},
    license_info={"name": "MIT"},
    openapi_tags=[
        {"name": "users", "description": "User management"},
        {"name": "items", "description": "Item CRUD"}
    ]
)

Tip: All params are optional. The bare app = FastAPI() works perfectly for development.

Minimal hello world - annotated

First app

from fastapi import FastAPI

app = FastAPI()               # create instance

@app.get("/")                  # register GET / route
async def root():
    return {"message": "Hello!"}  # dict -> JSON automatically

# FastAPI auto-handles:
# ✓ dict/list/Pydantic model -> JSON
# ✓ Content-Type: application/json
# ✓ /docs, /redoc, /openapi.json
# ✓ 422 validation on bad input

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App instance

Overview

Key concepts

Best practices

Creating the app

FastAPI() - all constructor params

Minimal hello world - annotated

Comments

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Backend

Frontend

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