Python & FastAPI Best Practices

Architectural patterns and performance guide for FastAPI, Pydantic v2, and SQLAlchemy 2.0 applications. Contains 38 rules across 8 categories, prioritized by impact. Focuses on patterns that linters cannot catch — async correctness, dependency injection, session management, schema design.

When to Apply

Reference these guidelines when:

• Writing new FastAPI endpoints or routers
• Designing Pydantic request/response models
• Implementing SQLAlchemy database queries or sessions
• Reviewing async Python code for correctness
• Refactoring existing FastAPI applications
• Setting up project structure for a new Python API

Target Versions

• Python 3.11+
• FastAPI 0.104+
• Pydantic 2.x
• SQLAlchemy 2.0+
• Alembic 1.12+

Rule Categories by Priority

Priority	Category	Impact	Prefix	Rules
1	Async Correctness	CRITICAL	async-	5
2	Dependency Injection	CRITICAL	di-	5
3	Database Patterns	HIGH	db-	6
4	Pydantic Models	HIGH	pydantic-	5
5	Error Handling	MEDIUM-HIGH	error-	4
6	API Design	MEDIUM	api-	5
7	Testing Patterns	MEDIUM	test-	4
8	Project Structure	LOW-MEDIUM	structure-	4

Quick Reference

1. Async Correctness (CRITICAL)

• async-no-blocking - Never call blocking I/O in async endpoints
• async-gather-parallel - Use asyncio.gather for independent async calls
• async-taskgroup - Use TaskGroup for structured concurrency with error handling
• async-connection-pool - Configure async connection pools, don't create per-request
• async-sync-in-threadpool - Run sync code in threadpool when unavoidable

2. Dependency Injection (CRITICAL)

• di-yield-dependencies - Use yield for setup/teardown lifecycle
• di-annotated-depends - Use Annotated[T, Depends()] for reusable type aliases
• di-caching - Understand use_cache behavior for request-scoped singletons
• di-class-dependencies - Group related dependencies in injectable classes
• di-no-global-state - Never use module-level mutable state, inject instead

3. Database Patterns (HIGH)

• db-session-dependency - Provide sessions via DI with proper commit/rollback
• db-n-plus-one - Prevent N+1 with selectinload/joinedload
• db-async-session - Use AsyncSession with async engine
• db-transaction-boundaries - Explicit transaction scoping per operation
• db-reversible-migrations - Every Alembic migration must have a downgrade
• db-select-api - Use select() API, not legacy Query

4. Pydantic Models (HIGH)

• pydantic-separate-schemas - Separate Create/Read/Update schemas per resource
• pydantic-field-validators - Use field_validator and model_validator correctly
• pydantic-base-settings - Use BaseSettings for environment configuration
• pydantic-model-config - Use model_config dict, not inner class Config
• pydantic-v2-serialization - Use model_dump/model_validate, not dict/parse_obj

5. Error Handling (MEDIUM-HIGH)

• error-exception-handlers - Register app-wide exception handlers
• error-structured-responses - Consistent error response schema
• error-http-exception - Use HTTPException with appropriate status codes
• error-validation-errors - Customize Pydantic validation error format

6. API Design (MEDIUM)

• api-router-organization - One router per domain with tags and prefixes
• api-response-model - Always declare response_model on endpoints
• api-lifespan - Use lifespan context manager, not on_event
• api-middleware-order - Understand middleware execution order (LIFO)
• api-versioning - Version APIs via URL prefix

7. Testing Patterns (MEDIUM)

• test-async-client - Use httpx.AsyncClient with ASGITransport
• test-dependency-overrides - Override dependencies for isolated testing
• test-factories - Use factory functions or factory_boy for test data
• test-db-isolation - Transaction rollback between tests for isolation

8. Project Structure (LOW-MEDIUM)

• structure-domain-packages - Organize by domain, not by layer
• structure-config-module - Centralize configuration in a settings module
• structure-init-exports - Use init.py for clean public APIs
• structure-alembic-setup - Configure Alembic with async support and naming conventions

Linting & Formatting

For Ruff, mypy, Black, and other tooling configuration, see the code-quality skill. This skill covers architectural patterns only.

Full Compiled Document

For the complete guide with all rules expanded and code examples: AGENTS.md