FastAPI

FastAPI

Overview

High-performance Python backend engineering using FastAPI, Pydantic v2, and async SQLAlchemy/Tortoise ORM. Enforces type-driven request validation, OpenAPI contracts, and async non-blocking endpoints.

When to Use

Activate when building Python REST APIs, microservices, asynchronous background jobs, or integrating Python ML services into web backends.

Rules & Patterns

FastAPI — Best Practices

Project Structure

app/
├── main.py              # App entry, CORS, middleware
├── config.py            # Settings with Pydantic BaseSettings
├── database.py          # Database session, engine
├── models/              # SQLAlchemy models
│   ├── __init__.py
│   └── user.py
├── schemas/             # Pydantic schemas (request/response)
│   ├── __init__.py
│   └── user.py
├── api/                 # Route handlers
│   ├── __init__.py
│   ├── deps.py          # Dependency injection
│   └── v1/
│       ├── __init__.py
│       └── users.py
├── services/            # Business logic
│   └── user_service.py
├── repositories/        # Database access
│   └── user_repo.py
└── tests/
    └── test_users.py

Pydantic Models

from pydantic import BaseModel, EmailStr, Field

class UserCreate(BaseModel):
    email: EmailStr
    name: str = Field(..., min_length=1, max_length=100)
    
class UserResponse(BaseModel):
    id: int
    email: str
    name: str
    
    model_config = ConfigDict(from_attributes=True)

Dependency Injection

from fastapi import Depends
from sqlalchemy.ext.asyncio import AsyncSession

async def get_db() -> AsyncGenerator[AsyncSession, None]:
    async with async_session() as session:
        yield session

async def get_current_user(
    token: str = Depends(oauth2_scheme),
    db: AsyncSession = Depends(get_db)
) -> User:
    # Verify token, return user
    ...

Async

• Use async for all I/O operations (database, HTTP calls, file I/O)
• Never block the event loop — no sync I/O in async endpoints
• Use asyncio.gather for parallel async operations
• Background tasks — BackgroundTasks for non-critical work

Error Handling

from fastapi import HTTPException

class AppException(HTTPException):
    def __init__(self, status_code: int, detail: str, code: str):
        super().__init__(status_code=status_code, detail=detail)
        self.code = code

Security

• OAuth2 with JWT — use python-jose
• Password hashing — bcrypt via passlib
• CORS — configure explicitly
• Rate limiting — use slowapi
• Input validation — Pydantic handles this automatically

Testing

import pytest
from httpx import AsyncClient

@pytest.mark.asyncio
async def test_create_user(client: AsyncClient):
    response = await client.post("/api/v1/users", json={
        "email": "test@example.com",
        "name": "Test User"
    })
    assert response.status_code == 201

Anti-Patterns

• [FAIL] Business logic in route handlers — use services
• [FAIL] Raw SQL without ORM — use SQLAlchemy
• [FAIL] Sync database calls — use async drivers
• [FAIL] Hardcoded settings — use Pydantic BaseSettings
• [FAIL] No schema validation — always use Pydantic models

Code Examples

See EXAMPLES.md for detailed code examples.

Validation Checklist

What to verify during the review phase before completing the task.

Common Mistakes

Anti-patterns and things to explicitly avoid. See TROUBLESHOOTING.md.

Integration Notes

How this skill interacts with other skills.

fastapi Examples — Anti-patterns vs ContextOS Standard

Example 1: Asynchronous Route Handlers

Anti-pattern: Blocking I/O inside async def

# BAD: time.sleep or synchronous requests blocks the entire asyncio event loop!
import time
import requests

@app.get("/slow")
async def slow_route():
    time.sleep(5)  # BLOCKS ALL CONCURRENT USERS!
    return {"status": "done"}

Best practice: ContextOS Standard (Non-blocking Async or Def Offload)

# GOOD: Use async non-blocking client (httpx) or standard def for sync CPU work
import asyncio
import httpx

@app.get("/fast")
async def fast_route():
    async with httpx.AsyncClient() as client:
        response = await client.get("https://api.example.com/data")
    return response.json()

# Or standard def (FastAPI automatically runs it in a background threadpool):
@app.get("/sync-worker")
def sync_worker():
    time.sleep(5)  # Runs in worker thread without blocking event loop
    return {"status": "done"}

fastapi Troubleshooting & Common Mistakes

1. Pydantic v1 vs v2 Deprecations

• Symptom: Warnings or crashes regarding @validator or .dict() methods.
• Root Cause: FastAPI projects upgrading to Pydantic v2.
• Fix: Use @field_validator instead of @validator, and .model_dump() instead of .dict().

2. Database Session Leaks

• Symptom: Database pool runs out of connections after a few requests.
• Root Cause: Database sessions opened manually without proper try...finally or dependency injection.
• Fix: Always provide database sessions via Depends(get_db) with a yield block.

3. Unhandled Validation Errors Returning Inconsistent JSON

• Symptom: Frontend receives raw 422 arrays without matching standard API error response envelope.
• Root Cause: Missing custom RequestValidationError handler.
• Fix: Register an app-level exception handler for RequestValidationError that normalizes error shapes.