python

# Python Backend Patterns

## Overview

Python patterns for building backend services with FastAPI.

## Project Structure

```
project/
├── app/
│   ├── __init__.py
│   ├── main.py               # FastAPI app
│   ├── config.py             # Configuration
│   ├── dependencies.py       # Dependency injection
│   ├── routers/              # API routes
│   │   ├── __init__.py
│   │   └── users.py
│   ├── services/             # Business logic
│   ├── repositories/         # Data access
│   ├── models/               # SQLAlchemy models
│   ├── schemas/              # Pydantic schemas
│   └── utils/                # Utilities
├── tests/                    # Test files
├── migrations/               # Alembic migrations
├── pyproject.toml
└── requirements.txt
```

## FastAPI Application

### Main Application

```python
# app/main.py
from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware
from contextlib import asynccontextmanager

from app.config import settings
from app.routers import users, auth
from app.database import engine, Base

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)
    yield
    # Shutdown
    await engine.dispose()

app = FastAPI(
    title="My API",
    version="1.0.0",
    lifespan=lifespan,
)

app.add_middleware(
    CORSMiddleware,
    allow_origins=settings.cors_origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

app.include_router(auth.router, prefix="/api/auth", tags=["auth"])
app.include_router(users.router, prefix="/api/users", tags=["users"])

@app.get("/health")
async def health():
    return {"status": "ok"}
```

### Configuration

```python
# app/config.py
from pydantic_settings import BaseSettings
from functools import lru_cache

class Settings(BaseSettings):
    database_url: str = "postgresql+asyncpg://localhost/app"
    redis_url: str = "redis://localhost:6379"
    secret_key: str = "your-secret-key"
    access_token_expire_minutes: int = 30
    cors_origins: list[str] = ["http://localhost:3000"]

    class Config:
        env_file = ".env"

@lru_cache
def get_settings() -> Settings:
    return Settings()

settings = get_settings()
```

## Pydantic Schemas

```python
# app/schemas/user.py
from pydantic import BaseModel, EmailStr, Field
from datetime import datetime
from typing import Optional

class UserBase(BaseModel):
    name: str = Field(..., min_length=2, max_length=100)
    email: EmailStr

class UserCreate(UserBase):
    password: str = Field(..., min_length=8)

class UserUpdate(BaseModel):
    name: Optional[str] = Field(None, min_length=2, max_length=100)
    email: Optional[EmailStr] = None

class UserResponse(UserBase):
    id: str
    created_at: datetime

    class Config:
        from_attributes = True

class PaginatedResponse(BaseModel):
    items: list[UserResponse]
    total: int
    page: int
    page_size: int
```

## SQLAlchemy Models

```python
# app/models/user.py
from sqlalchemy import Column, String, DateTime, Boolean
from sqlalchemy.sql import func
from app.database import Base
import uuid

class User(Base):
    __tablename__ = "users"

    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
    name = Column(String(100), nullable=False)
    email = Column(String(255), unique=True, nullable=False, index=True)
    password_hash = Column(String(255), nullable=False)
    is_active = Column(Boolean, default=True)
    created_at = Column(DateTime(timezone=True), server_default=func.now())
    updated_at = Column(DateTime(timezone=True), onupdate=func.now())
```

## Async Database

```python
# app/database.py
from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
from sqlalchemy.orm import sessionmaker, declarative_base
from app.config import settings

engine = create_async_engine(settings.database_url, echo=True)
async_session = sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
Base = declarative_base()

async def get_db():
    async with async_session() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise
        finally:
            await session.close()
```

## Repository Pattern

```python
# app/repositories/user.py
from sqlalchemy import select, func
from sqlalchemy.ext.asyncio import AsyncSession
from app.models.user import User
from app.schemas.user import UserCreate, UserUpdate

class UserRepository:
    def __init__(self, session: AsyncSession):
        self.session = session

    async def find_by_id(self, id: str) -> User | None:
        result = await self.session.execute(
            select(User).where(User.id == id)
        )
        return result.scalar_one_or_none()

    async def find_by_email(self, email: str) -> User | None:
        result = await self.session.execute(
            select(User).where(User.email == email)
        )
        return result.scalar_one_or_none()

    async def find_all(self, page: int = 1, page_size: int = 20) -> tuple[list[User], int]:
        offset = (page - 1) * page_size

        # Get items
        result = await self.session.execute(
            select(User)
            .order_by(User.created_at.desc())
            .offset(offset)
            .limit(page_size)
        )
        items = result.scalars().all()

        # Get total count
        count_result = await self.session.execute(
            select(func.count()).select_from(User)
        )
        total = count_result.scalar()

        return list(items), total

    async def create(self, data: UserCreate, password_hash: str) -> User:
        user = User(
            name=data.name,
            email=data.email,
            password_hash=password_hash,
        )
        self.session.add(user)
        await self.session.flush()
        return user

    async def update(self, user: User, data: UserUpd