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name: fastapi-development description: Enforces best practices for FastAPI development, ensuring scalable, high-performance, and maintainable APIs. Use when working on server-side Python code involving FastAPI.

FastAPI Development Standards

Key Principles

• Concise & Technical: Write accurate, functional code.
• Functional Style: Prefer pure functions over classes for routes and logic.
• RORO Pattern: "Receive an Object, Return an Object". Use Pydantic models for all inputs and outputs.

Architecture & Structure

• File Naming: Lowercase with underscores (e.g., routers/user_routes.py).
• Exports: Favor named exports.
• Organization: Separate exported routers, sub-routes, utilities, static content, and types (models/schemas).

Coding Standards

• Type Hints: Mandatory for all function signatures.
• Validation: Use Pydantic models (v2) instead of raw dictionaries.
• Conditionals: Avoid unnecessary curly braces. Use one-line syntax for simple conditionals.
• Async/Sync:
  • Use async def for I/O-bound operations (DB, external APIs).
  • Use def for CPU-bound synchronous tasks.

Error Handling

• Early Returns: Handle errors/edge cases first. Happy path goes last.
• Guard Clauses: Validate preconditions early.
• HTTPExceptions: Use HTTPException for expected errors.
• No Nested Ifs: Avoid deep nesting.

Performance

• Non-blocking: Minimize blocking I/O. Use async libraries (asyncpg, aiomysql).
• Caching: Implement strategies for frequent data.
• Lazy Loading: Use for large datasets.

Dependencies

• FastAPI
• Pydantic v2
• SQLAlchemy 2.0 (if ORM used)