FastAPI Patterns

Async Traps

• Mixing sync database drivers (psycopg2, PyMySQL) in async endpoints blocks the event loop — use async drivers (asyncpg, aiomysql) or run sync code in run_in_executor
• time.sleep() in async endpoints blocks everything — use await asyncio.sleep() instead
• CPU-bound work in async endpoints starves other requests — offload to ProcessPoolExecutor or background workers
• Async endpoints calling sync functions that do I/O still block — the entire call chain must be async

Pydantic Validation

• Default values in models become shared mutable state: items: list = [] shares the same list across requests — use Field(default_factory=list)
• Optional[str] doesn't make a field optional in the request — add = None or use Field(default=None)
• Pydantic v2 uses model_validate() not parse_obj(), and model_dump() not .dict() — v1 methods are deprecated
• Use Annotated[str, Field(min_length=1)] for reusable validated types instead of repeating constraints

Dependency Injection

• Dependencies run on every request by default — use lru_cache on expensive dependencies or cache in app.state for singletons
• Depends() without an argument reuses the type hint as the dependency — clean but can confuse readers
• Nested dependencies form a DAG — if A depends on B and C, and both B and C depend on D, D runs once (cached per-request)
• yield dependencies for cleanup (DB sessions, file handles) — code after yield runs even if the endpoint raises