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FastAPI & API Design

FastAPI request flow, dependencies, validation, errors, sync and async handlers, and reliable HTTP API boundaries.

34 drillsPractice one at a timeOrdered pathConcept questionComparison questionArchitecture scenarioDebugging scenario

Recommended start

What FastAPI provides

Defines FastAPI as an ASGI web framework and explains how types drive validation, dependency resolution, responses, and OpenAPI.

Easy~5 minStep 1 of 34

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1What FastAPI providesRecommended startDefines FastAPI as an ASGI web framework and explains how types drive validation, dependency resolution, responses, and OpenAPI.Concept questionEasy~5 minFastAPIPydanticPythonPractice 2

Practice order

34 drillsOne at a time

1What FastAPI providesUp next / Easy / ~5 min
2FastAPI request lifecycleMedium / ~7 min
3FastAPI parameter sourcesMedium / ~7 min
4

FastAPI request lifecycleTraces a request from the ASGI server through middleware, routing, dependencies, validation, handler execution, and response completion.Concept questionMedium~7 minFastAPIPydanticPython

Practice

3FastAPI parameter sourcesExplains FastAPI parameter-source inference and when explicit markers make an HTTP contract clearer.Concept questionMedium~7 minFastAPIPydanticPythonPractice

4HTTP content negotiationDistinguishes request and response media types and designs predictable negotiation, rejection, documentation, and caching behavior.Concept questionMedium~7 min.NETASP.NET CorePractice

5`def` vs `async def`Chooses between synchronous and asynchronous FastAPI handlers based on the libraries and work they execute.Comparison questionMedium~7 minFastAPIPydanticPythonPractice

6FastAPI dependency graphExplains dependency graphs, sub-dependencies, request-local caching, failures, and deliberate cache bypass.Concept questionMedium~7 minFastAPIPydanticPythonPractice

7`yield` dependency lifetimeExplains acquisition and cleanup around `yield`, including request and function scopes, streaming, and exception-safe ownership.Architecture scenarioHard~10 minFastAPIPydanticPythonPractice

8Keep `Depends` at the boundaryKeeps FastAPI dependency resolution at transport and composition boundaries while application services use explicit dependencies.Concept questionHard~10 minFastAPIPydanticPythonPractice

9Organize with `APIRouter`Uses routers as feature-oriented HTTP modules while keeping application logic, composition, and contract ownership clear.Architecture scenarioMedium~7 minFastAPIPydanticPythonPractice

10Pydantic request boundaryUses Pydantic models as explicit transport contracts without making them persistence or domain models automatically.Architecture scenarioMedium~7 minFastAPIPydanticPythonPractice

11Request DTOsDesign request DTOs that protect API contracts from domain and persistence models.Architecture scenarioMedium~10 min.NETASP.NET CoreC#Practice

12Pydantic coercion and strictnessExplains how input conversion, strict validation, and extra-field policies change what an API accepts and preserves.Concept questionMedium~7 minFastAPIPydanticPythonPractice

13Validators vs domain rulesSeparates deterministic request-shape validation from authorization, I/O, workflows, and durable business invariants.Concept questionMedium~7 minFastAPIPydanticPythonPractice

14Pydantic discriminated unionsUses an explicit discriminator to model genuine request variants with predictable validation and OpenAPI output.Concept questionHard~10 minFastAPIPydanticPythonPractice

15Missing vs null in PATCHModels partial updates without confusing omitted fields, explicit nulls, defaults, or unauthorized changes.Architecture scenarioMedium~7 minFastAPIPydanticPythonPractice

16FastAPI response modelsExplains runtime output validation, filtering, serialization, and the security value of an explicit response contract.Concept questionMedium~7 minFastAPIPydanticPythonPractice

17Return data vs `Response`Chooses between FastAPI-managed response processing and direct control over the HTTP response.Concept questionMedium~7 minFastAPIPydanticPythonPractice

18Choose HTTP status codesMaps creation, updates, asynchronous work, validation, authorization, conflicts, and failures to useful HTTP outcomes.Architecture scenarioMedium~10 min.NETASP.NET CorePractice

19Request rejected before handlerLocates pre-handler failures across proxies, routing, parsing, validation, dependencies, and middleware using structured evidence.Debugging scenarioHard~10 minFastAPIPydanticPythonPractice

20FastAPI error contractMaps validation, HTTP, domain, and unexpected failures into a stable client contract while preserving diagnostics.Architecture scenarioHard~10 minFastAPIPydanticPythonPractice

21ProblemDetails vs Custom ErrorsUses RFC 9457 Problem Details as a stable, safe error contract with documented problem types, actionable extensions, and centralized ASP.NET mapping.Comparison questionMedium~7 min.NETASP.NET CoreC#Practice

22FastAPI middleware orderExplains nested ASGI middleware ordering, reverse response flow, exception boundaries, CORS, and streaming-safe design.Architecture scenarioHard~10 minFastAPIPydanticPythonPractice

23CORS and preflightExplains browser same-origin enforcement, preflight requests, credentials, middleware behavior, and why CORS is not authorization.Concept questionMedium~7 min.NETASP.NET CorePractice

24CORS BoundaryExplains what CORS controls, what it does not protect, and why APIs still need authentication and authorization.Concept questionEasy~5 min.NETASP.NET CoreJWTPractice

25FastAPI lifespan ownershipUses lifespan to acquire and release process-level resources while making readiness, workers, failures, and tests explicit.Architecture scenarioHard~10 minFastAPIPydanticPythonPractice

26`BackgroundTasks` vs job queueDistinguishes small in-process post-response work from durable, retryable, independently operated jobs.Concept questionHard~10 minFastAPIPydanticPythonPractice

27Idempotent POSTMakes POST retries return one durable business outcome by combining scoped keys, request identity, atomic ownership, response replay, and reconciliation.Architecture scenarioHard~12 min.NETASP.NET CoreC#Practice

28Safe FastAPI streamingStreams bounded chunks with explicit resource lifetime, cancellation, late-failure, and proxy behavior.Architecture scenarioHard~10 minFastAPIPydanticPythonPractice

29Safe FastAPI file uploadsHandles multipart uploads with bounded size, untrusted metadata, content inspection, controlled storage, scanning, and cleanup.Architecture scenarioHard~10 minFastAPIPydanticPythonPractice

30Request body boundariesHandles the request body as a forward-only resource, buffers only when necessary, and enforces limits before large input consumes the service.Architecture scenarioHard~12 min.NETASP.NET CoreC#Practice

31Pagination & FilteringDesign list endpoints with pagination, filtering, sorting, limits, and database-aware performance behavior.Architecture scenarioMedium~10 min.NETASP.NET CoreC#Practice

32Control FastAPI OpenAPIControls operation identities, request and response schemas, error documentation, and compatibility checks without hand-maintaining a second contract.Architecture scenarioHard~10 minFastAPIPydanticPythonPractice

33API Contract TestsCombines schema diffing, real provider tests, consumer expectations, generated clients, semantic scenarios, and rollout evidence to prevent accidental API breaks.Concept questionMedium~7 min.NETASP.NET CoreC#Practice

34API vs App VersioningSeparates deployable artifact identity from client-facing contract compatibility, allowing frequent releases without unnecessary API-version churn.Comparison questionMedium~7 min.NETASP.NET CoreGitPractice

HTTP content negotiation

Medium / ~7 min

5`def` vs `async def`Medium / ~7 min

6FastAPI dependency graphMedium / ~7 min

7`yield` dependency lifetimeHard / ~10 min

8Keep `Depends` at the boundaryHard / ~10 min

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FastAPI & API Design - Python Backend | Aporeon