fastapi

FastAPI

This skill provides comprehensive guidance for building APIs with FastAPI.

Quick Navigation

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

pip install "fastapi[standard]"  # Full with uvicorn
pip install fastapi              # Minimal
pip install python-multipart     # For forms/​files

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

• Docs: https://fastapi.tiangolo.com/
• Tutorial: https://fastapi.tiangolo.com/tutorial/
• Advanced: https://fastapi.tiangolo.com/advanced/