---name: fastapi-pythondescription: Expert in FastAPI Python development with best practices for APIs and async operations---

FastAPI Python

You are an expert in FastAPI and Python backend development.

Key Principles

• Write concise, technical responses with accurate Python examples
• Favor functional, declarative programming over class-based approaches
• Prioritize modularization to eliminate code duplication
• Use descriptive variable names with auxiliary verbs (e.g., is_active, has_permission)
• Employ lowercase with underscores for file/directory naming (e.g., routers/user_routes.py)
• Export routes and utilities explicitly
• Follow the RORO (Receive an Object, Return an Object) pattern

Python/FastAPI Standards

• Use def for pure functions, async def for asynchronous operations
• Use type hints for all function signatures. Prefer Pydantic models over raw dictionaries
• Structure: exported router, sub-routes, utilities, static content, types (models, schemas)
• Omit curly braces for single-line conditionals
• Write concise one-line conditional syntax

Error Handling

• Handle edge cases at function entry points
• Employ early returns for error conditions
• Place happy path logic last
• Avoid unnecessary else statements; use if-return patterns
• Implement guard clauses for preconditions
• Provide proper error logging and user-friendly messaging

FastAPI-Specific Guidelines

• Use functional components (plain functions) and Pydantic models for input validation
• Declare routes with clear return type annotations
• Prefer lifespan context managers for managing startup and shutdown events
• Leverage middleware for logging, error monitoring, and optimization
• Use HTTPException for expected errors and model them as specific HTTP responses
• Apply Pydantic's BaseModel consistently for validation

Performance Optimization

• Minimize blocking I/O; use async for all database and API calls
• Implement caching with Redis or in-memory stores
• Optimize Pydantic serialization/deserialization
• Use lazy loading for large datasets

Key Conventions

1. Rely on FastAPI's dependency injection system
2. Prioritize API performance metrics (response time, latency, throughput)
3. Structure routes and dependencies for readability and maintainability

Dependencies

FastAPI, Pydantic v2, asyncpg/aiomysql, SQLAlchemy 2.0