Python Backend Patterns Skill

Purpose

Best practices for Python backend development with FastAPI, SQLAlchemy, and async programming.

Auto-Invoke Triggers

• •Creating FastAPI routes
• •Working with SQLAlchemy models
• •Implementing async operations
• •Creating Pydantic schemas

Project Structure

backend/
├── app/
│   ├── main.py              # FastAPI app initialization
│   ├── config.py            # Settings (pydantic-settings)
│   ├── api/v1/endpoints/    # Route handlers
│   ├── core/                # Security, exceptions
│   ├── models/              # SQLAlchemy models
│   ├── schemas/             # Pydantic schemas
│   ├── services/            # Business logic
│   ├── repositories/        # Data access
│   └── db/session.py        # Database session
├── tests/
├── alembic.ini
└── requirements.txt

Layer Responsibilities

Async Database Patterns

Session Management

• •Use async_sessionmaker for async sessions
• •Use dependency injection for session
• •Commit in dependency, rollback on exception
• •Use expire_on_commit=False for response data

Query Patterns

• •Use select() statements (SQLAlchemy 2.0 style)
• •Prefer scalar_one_or_none() for single items
• •Use scalars().all() for lists
• •Eager load relationships with selectin or joinedload

Pydantic Schema Patterns

Schema Types

Best Practices

• •Use model_config = ConfigDict(from_attributes=True) for ORM
• •Use Field() for validation constraints
• •Use field_validator for custom validation
• •Separate request and response schemas

Repository Pattern

Base Repository Methods

• •get_by_id(id) - Single item by primary key
• •get_all(skip, limit) - Paginated list
• •create(**kwargs) - Insert new record
• •update(id, **kwargs) - Update existing
• •delete(id) - Remove record

Specific Repositories

• •Extend base with domain-specific queries
• •Example: get_by_email(), get_active_users()

Service Layer

Responsibilities

• •Validate business rules
• •Coordinate multiple repositories
• •Transform data between layers
• •Raise domain exceptions

Pattern

• •Inject repository via constructor
• •Return Pydantic schemas, not models
• •Raise specific exceptions (NotFoundError, ConflictError)

Dependency Injection

Common Dependencies

• •get_db - Database session
• •get_current_user - Authenticated user
• •get_current_superuser - Admin user
• •Service factories - get_user_service(session)

Pattern

# Endpoint receives dependencies, passes to service
async def create_user(
    data: UserCreate,
    session: AsyncSession = Depends(get_db)
):
    service = UserService(session)
    return await service.create(data)

Configuration

Settings Class

• •Use pydantic-settings for env loading
• •Use @lru_cache for singleton
• •Define defaults for optional settings
• •Use @property for computed values

Environment Variables

• •DATABASE_URL - Database connection
• •SECRET_KEY - JWT signing
• •DEBUG - Development mode
• •CORS_ORIGINS - Allowed origins

Testing Patterns

Fixtures

• •db_session - In-memory SQLite session
• •client - AsyncClient with app
• •Override get_db dependency for tests

Test Structure

• •One test file per module
• •Use pytest.mark.asyncio for async tests
• •AAA pattern: Arrange, Act, Assert
• •Mock external services

Best Practices

DO

• •Use async/await consistently
• •Use type hints everywhere
• •Use Pydantic for all validation
• •Use dependency injection
• •Use repository pattern for data access
• •Separate business logic into services

DON'T

• •Mix sync and async database calls
• •Put business logic in routes
• •Use raw SQL without parameters
• •Catch generic Exception
• •Store secrets in code
• •Skip server-side validation

Code Quality