FastAPI Interview Questions — Medium

• ASGI defines a single callable async def app(scope, receive, send) handling HTTP/WebSocket/lifespan messages.
• Lifespan events (@app.on_event("startup") legacy / lifespan context manager modern) initialize pools (DB, Redis, httpx clients) once per process - dependencies then reuse them.
• Uvicorn manages the event loop; multiple worker processes each run the full lifespan. Interview depth: explain why per-request engine creation is an anti-pattern vs lifespan-held pools.

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• Within ONE request, identical dependency (by callable identity + params) resolves once - Depends(get_db) in handler + sub-dependency shares the session.
• Bust via use_cache=False on Depends when you genuinely need fresh resolution (per-item security checks in loops).
• Global scope alternative: module-level singletons for cross-request sharing (connection pools) - DI is for per-request composition.

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async def get_db():
    async with async_session() as s:
        yield s

• Code before yield runs pre-handler; after-yield executes AFTER response completes (exit of context), enabling commit/rollback decisions based on exceptions propagated from handlers.
• Caveat: background tasks using that dependency may outlive - FastAPI handles ordering but long tasks should open own sessions. Exception injection: raising inside post-yield wraps handler errors - understand interplay with custom exception middleware.

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class RoleChecker:
    def __init__(self, *roles): self.roles = set(roles)
    def __call__(self, user: User = Depends(get_current_user)):
        if user.role not in self.roles: raise HTTPException(403)

@router.post("/admin", dependencies=[Depends(RoleChecker("admin"))])

• Callable instances become parameterized dependencies - reusable guards.
• OAuth2 scopes: Security(get_current_user, scopes=["items:write"]); token contains scopes; SecurityScopes dependency verifies hierarchy - maps to OpenAPI security UI.

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@field_validator("email")
@classmethod
def normalize(cls, v): return v.lower()

@model_validator(mode="after")
def check_dates(self):
    if self.end < self.start: raise ValueError(...)

• field_validator runs per-field pre/post coercion (mode="before" sees raw input).
• model_validator "before" receives raw dict (cross-field normalization), "after" receives constructed model instance. Return values replace data - forgetting to return in before-validators silently drops fields (classic bug).

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