FastAPI

This skill provides comprehensive guidance for building APIs with FastAPI.

Quick Navigation

Topic	Reference
Getting started	references/first-steps.md
Path parameters	references/path-parameters.md
Query parameters	references/query-parameters.md
Request body	references/request-body.md
Validation	references/validation.md
Body advanced	references/body-advanced.md
Cookies/Headers	references/cookies-headers.md
Pydantic models	references/models.md
Forms/Files	references/forms-files.md
Error handling	references/error-handling.md
Path config	references/path-config.md
Dependencies	references/dependencies.md
Security	references/security.md
Middleware	references/middleware.md
CORS	references/cors.md
Database	references/sql-databases.md
Project structure	references/bigger-applications.md
Background tasks	references/background-tasks.md
Metadata/Docs	references/metadata-docs.md
Testing	references/testing.md
Advanced responses	references/responses-advanced.md
WebSockets	references/websockets.md
Templates	references/templates.md
Settings/Env vars	references/settings.md
Lifespan events	references/lifespan.md
OpenAPI advanced	references/openapi-advanced.md

When to Use

• Creating REST APIs with Python
• Adding endpoints with automatic validation
• Implementing OAuth2/JWT authentication
• Working with Pydantic models
• Adding dependency injection
• Configuring CORS, middleware
• Uploading files, handling forms
• Testing API endpoints

Installation

Requires Python 3.10+. Install: pip install "fastapi[standard]" (full with uvicorn) or pip install fastapi (minimal). Add python-multipart for forms/files.

Release Highlights (0.133.0 → 0.136.1)

• 0.134.0: streaming JSON Lines and streaming binary data support using yield.
• 0.135.0: first-class Server-Sent Events (SSE) support (EventSourceResponse).
• 0.135.1: fix around TaskGroup usage in request async exit stack (stability fix).
• 0.136.1: FastAPI updates its Pydantic v2 code to avoid deprecations and bumps Starlette to 1.0.0.

Patch Notes (0.136.2 → 0.136.3)

• SSE responses now validate event fields more strictly, so malformed ServerSentEvent payloads fail earlier instead of quietly streaming invalid frames.
• Header parameters no longer accept underscore-named incoming headers when convert_underscores=True (the default). If a client truly sends underscore headers, declare Header(convert_underscores=False) and verify that your proxy chain allows them.

Quick Start

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

Run: fastapi dev main.py

Core Patterns

Type-Safe Parameters

from typing import Annotated
from fastapi import Path, Query

@app.get("/items/{item_id}")
def read_item(
    item_id: Annotated[int, Path(ge=1)],
    q: Annotated[str | None, Query(max_length=50)] = None
):
    return {"item_id": item_id, "q": q}

Request Body with Validation

from pydantic import BaseModel, Field

class Item(BaseModel):
    name: str = Field(min_length=1, max_length=100)
    price: float = Field(gt=0)

@app.post("/items/", response_model=Item)
def create_item(item: Item):
    return item

Dependencies

from fastapi import Depends

async def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

@app.get("/users/")
def list_users(db: Annotated[Session, Depends(get_db)]):
    return db.query(User).all()

Authentication

from fastapi.security import OAuth2PasswordBearer

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

async def get_current_user(token: Annotated[str, Depends(oauth2_scheme)]):
    return decode_token(token)

@app.get("/users/me")
def read_me(user: Annotated[User, Depends(get_current_user)]):
    return user

API Documentation

• Swagger UI: /docs
• ReDoc: /redoc
• OpenAPI: /openapi.json

Best Practices

• Use Annotated[Type, ...] for parameters
• Define Pydantic models for request/response
• Use response_model for output filtering
• Add status_code for proper HTTP codes
• Use tags for API organization
• Add dependencies at router/app level for auth

Prohibitions

• ❌ Return raw database models (use response models)
• ❌ Store passwords in plain text (use bcrypt/passlib)
• ❌ Mix Body with Form/File in same endpoint
• ❌ Use sync blocking I/O in async endpoints
• ❌ Skip HTTPException for error handling

Links

• Documentation
• Releases
• GitHub
• PyPI