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Top 30 Fastapi Interview Questions and Answers

30 multiple-choice questions on Fastapi, drawn from 30 bites out of the 211 tagged Fastapi on Tezvyn. Answer them here or read straight down. Every question carries the correct option, why it is correct, and a link to the bite it came from.

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  1. Question 1 of 30

    What is the primary mechanism by which Python type hints improve code reliability?

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    Answer: c · They enable static analysis tools to identify potential type mismatches before execution.

    Type hints are ignored by the Python interpreter at runtime. Their primary role is to allow static analysis tools to check for type consistency and potential errors before the code is ever executed, improving reliability. They do not perform runtime validation of external data.

    Read the full bite: Python Type Hints: Documentation Your Linter Can Read

  2. Question 2 of 30

    When you annotate a FastAPI route parameter with a Pydantic model, what does the framework do with that type hint?

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    Answer: a · It leverages the hint for automatic request validation and OpenAPI schema generation

    FastAPI reads type hints at startup to construct Pydantic models that validate incoming requests and generate OpenAPI schemas automatically. The distractor about runtime enforcement is wrong because Python itself ignores type hints during execution unless an external tool checks them.

    Read the full bite: Explain Python type hints and their importance in FastAPI

  3. Question 3 of 30

    You write an async def FastAPI endpoint that calls requests.get. What is the main risk?

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    Answer: c · The event loop is blocked, freezing concurrent request handling until the call finishes.

    The correct answer is C because calling a blocking library like requests inside async def stalls the event loop, stopping all other requests. The most tempting distractor is A because beginners often assume FastAPI magically threadpools any blocking code, but only def endpoints are run in a threadpool.

    Read the full bite: What is the difference between def and async def in Python and FastAPI?

  4. Question 4 of 30

    What mechanism triggers FastAPI to automatically validate and parse an incoming JSON request body against a schema?

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    Answer: b · Using a Pydantic model as the type hint for a route parameter

    FastAPI inspects function signature type hints at runtime, so using a Pydantic model as a parameter type hint automatically triggers request parsing and validation. Manually calling json.loads inside the route is a red flag that ignores this declarative mechanism, and response_model governs response serialization, not request validation.

    Read the full bite: How does FastAPI leverage Pydantic for request validation and serialization?

  5. Question 5 of 30

    When building an async execution-time decorator for FastAPI, why is omitting functools.wraps on the inner wrapper considered a red flag?

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    Answer: b · It strips the original function's metadata, which breaks FastAPI's OpenAPI schema generation and dependency injection.

    functools.wraps preserves the original function's name, signature, and metadata, which FastAPI relies on to generate OpenAPI docs and resolve dependencies; omitting it exposes the wrapper's metadata instead. Distractor A is wrong because wraps has no effect on whether code runs synchronously or blocks the event loop.

    Read the full bite: Write an async decorator that logs execution time for FastAPI

  6. Question 6 of 30

    Which implementation correctly minimizes total latency when a FastAPI endpoint must fetch data from two independent external APIs?

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    Answer: a · Inside an async def endpoint, pass two httpx.AsyncClient coroutines to asyncio.gather and await the result.

    asyncio.gather with an async HTTP client schedules both I/O-bound coroutines concurrently on the event loop, reducing total latency to roughly the slower call. Option C is tempting because it uses async/await correctly, but sequential awaiting means the second request cannot start until the first finishes, so latencies add up.

    Read the full bite: How do you structure concurrent API calls with asyncio.gather in FastAPI?

  7. Question 7 of 30

    When a decorator is applied to a function, what is the immediate effect on that function's definition?

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    Answer: d · It replaces the original function in its scope with a new, wrapped function.

    The card states that the decorator returns a wrapper function, which 'replaces the original my_func in the surrounding scope.' This means the original function's name now refers to the new, wrapped function. Option A is incorrect because decorators add behavior without altering the original function's source code.

    Read the full bite: Python Decorators: Functions that Wrap Functions

  8. Question 8 of 30

    When a FastAPI endpoint awaiting asyncpg is suspended during a database query, how can Uvicorn process another incoming connection in the same worker process?

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    Answer: d · The event loop yields the coroutine at the await, registers the socket with epoll or kqueue, and schedules the new connection's coroutine on the same thread.

    The correct answer describes cooperative multitasking: the event loop suspends the coroutine at await and interleaves I/O-bound tasks on a single thread. Distractor A is wrong because Uvicorn does not use multiple Python threads to handle requests; concurrency comes from the loop scheduling coroutines, not from threading or GIL behavior.

    Read the full bite: How does Uvicorn use asyncio to handle thousands of concurrent connections?

  9. Question 9 of 30

    In FastAPI, why should an async database session dependency wrap yield in try and place await session.close() in finally?

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    Answer: a · It guarantees cleanup runs even if the path operation raises an exception.

    A try/finally block guarantees that await session.close() runs even when the path operation raises an exception, preventing database connection leaks. The thread pool issue in distractor C is caused by using def instead of async def, not by omitting exception handling.

    Read the full bite: Implement an async database session dependency using yield for setup and teardown

  10. Question 10 of 30

    In Pydantic v2, what is the runtime validation behavior of a generic wrapper field typed as T when the model is used without parametrization?

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    Answer: a · The field is validated as Any, accepting arbitrary data and generating an overly permissive schema

    The card explicitly states that unparametrized TypeVars are treated as Any at validation time, yielding an overly permissive OpenAPI schema. Option C is a tempting distractor because developers often assume missing generic parameters cause runtime errors, but Pydantic v2 gracefully falls back to Any instead.

    Read the full bite: Create a generic Pydantic BaseModel for API response wrappers

  11. Question 11 of 30

    When an incoming GET request reaches a FastAPI app, how does @app.get("/") enable the correct function to run?

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    Answer: d · It registers the function in the app's route table at import time and generates OpenAPI metadata.

    The decorator actively registers the function for GET / in the app's internal route table when the module is imported and simultaneously populates the OpenAPI schema, enabling the ASGI layer to dispatch matching requests. Calling it pure syntax sugar is a common misconception because it fundamentally alters the application's routing registry and automatic documentation rather than leaving framework behavior unchanged.

    Read the full bite: What is the purpose of @app.get("/") in FastAPI?

  12. Question 12 of 30

    Which approach correctly defines a FastAPI endpoint that captures an integer item_id from a URL like /items/42?

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    Answer: a · Use app.get("/items/{item_id}") and define async def read_item(item_id: int): then use item_id directly inside the function

    FastAPI binds curly-braced path segments to function arguments with matching names and type hints, automatically converting and injecting the value. Option B is tempting because the route syntax is correct, but the mismatched argument name breaks the default binding unless you use a Path alias.

    Read the full bite: How do you define and access a FastAPI path parameter?

  13. Question 13 of 30

    You write an async generator that awaits during resource teardown, but mistakenly decorate it with @contextmanager. What is the most likely result?

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    Answer: b · A runtime error occurs because the synchronous decorator cannot await inside the generator

    The card explicitly warns that @contextmanager is for synchronous generators only and cannot await, causing a crash. Distractor A is tempting because blocking the event loop is discussed as a general risk of async code, but the specific mistake of using the sync decorator with async cleanup results in a runtime error rather than silent blocking.

    Read the full bite: Python Async Context Managers

  14. Question 14 of 30

    Which FastAPI endpoint declaration identifies a user in the URL path and accepts an optional search term after the question mark?

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    Answer: a · Route /users/{user_id}/items with user_id in path and q optional

    Option A correctly places the user identifier in the path template and gives q a default of None so it becomes an optional query parameter. Option B is tempting because it also makes q optional, but it omits user_id from the path so FastAPI treats it as a query parameter, violating RESTful hierarchy.

    Read the full bite: What is the difference between a path parameter and a query parameter?

  15. Question 15 of 30

    What specific problem did direct access to "__annotations__" for a class, like "Derived.__annotations__", commonly cause in Python 3.9 that "inspect.get_annotations()" addresses in Python 3.10+?

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    Answer: c · It would incorrectly show annotations inherited from a parent class, even if the derived class had no explicit annotations.

    In Python 3.9 and earlier, direct access to a class's __annotations__ would incorrectly include annotations inherited from parent classes. Python 3.10+ and `inspect.get_annotations()` resolve this by ensuring a class's __annotations__ only contains its own explicitly defined annotations, returning an empty dictionary if none are present.

    Read the full bite: Accessing Python Type Annotations Safely

  16. Question 16 of 30

    An endpoint declares a query parameter as q: Optional[str] with no default value. What happens when a client omits it?

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    Answer: c · FastAPI returns a 422 Unprocessable Entity error

    FastAPI derives requirement from the presence or absence of a Python signature default, not from type hints, so Optional[str] without = None is still required and omitting it triggers a 422 error. Option D is a common misconception because Optional alone does not make a parameter optional in FastAPI.

    Read the full bite: How does FastAPI distinguish required optional and default query parameters

  17. Question 17 of 30

    What combination of standard and code sources enables FastAPI's automatic interactive documentation?

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    Answer: d · It dynamically builds an OpenAPI schema from type hints, Pydantic models, decorators, and docstrings

    FastAPI dynamically generates an OpenAPI schema by extracting metadata from type hints, Pydantic models, decorators, and docstrings, so no manual schema file is required. Option B is wrong because maintaining a separate openapi.yaml by hand is unnecessary and contradicts FastAPI's design.

    Read the full bite: What standard and code elements power FastAPI's auto-generated API docs?

  18. Question 18 of 30

    What is the primary reason for creating a single FastAPI instance (e.g., app = FastAPI()) for your application?

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    Answer: d · It serves as the central manager for all routing, configuration, and lifecycle events.

    The FastAPI instance acts as the central, authoritative object that orchestrates the entire application, managing all routing, configuration, and lifecycle events. While it does contribute to API documentation, that is a feature derived from its central management role, not its primary purpose for being a single instance.

    Read the full bite: FastAPI Application Instance: Your API's Central Hub

  19. Question 19 of 30

    A client sends GET /items?limit=foo to an endpoint with parameter limit: int. What is FastAPI's default response?

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    Answer: a · HTTP 422 Unprocessable Entity with a JSON body whose detail array contains objects with loc, msg, and type fields

    FastAPI relies on Pydantic to automatically validate query parameters and returns a 422 Unprocessable Entity with a JSON detail array of objects containing loc, msg, and type fields. Option B is tempting because the status code is correct, but the body structure is actually a detailed array rather than a single string.

    Read the full bite: FastAPI non-integer query param default behavior

  20. Question 20 of 30

    Which scenario represents an inappropriate use of a FastAPI path operation decorator's arguments?

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    Answer: c · Dynamically changing the HTTP status code based on specific input conditions.

    FastAPI decorator arguments are designed for static configuration, such as setting a default status code or grouping endpoints. Dynamic logic, like returning different status codes based on request input, should be handled within the endpoint function itself, not via decorator arguments.

    Read the full bite: FastAPI: Configure Endpoints with Decorators

  21. Question 21 of 30

    To accept multiple values for a single query key and allow per-item length constraints in current FastAPI, which parameter declaration should you use?

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    Answer: c · tag: Annotated[list[str], Query()] = []

    Annotated[list[str], Query()] = [] is the modern pattern that separates validation metadata from the default value, enabling per-item constraints. tag: list[str] = Query(default=[]) is the outdated alternative that mixes the default with validation metadata.

    Read the full bite: How do you type-hint repeated query params in FastAPI?

  22. Question 22 of 30

    In FastAPI, what is the primary consequence of defining a path parameter without a type hint (e.g., item_id instead of item_id: int)?

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    Answer: c · The path parameter will always be treated as a string, regardless of its content.

    The card explicitly states that 'without int, 123 is just a string,' meaning FastAPI treats untyped path parameters as strings by default. FastAPI does not infer types for path parameters; it requires explicit type hints for validation and conversion.

    Read the full bite: Path Parameters: Turning URL Parts into Variables

  23. Question 23 of 30

    Which statement accurately describes the behavior of {file_path:path} compared to {file_path} in a FastAPI route definition?

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    Answer: c · {file_path:path} uses a Starlette converter to greedily match slashes across segments while {file_path} stops at the next slash

    {file_path:path} relies on Starlette's path converter to consume the rest of the URL including slashes, whereas a plain parameter matches only one segment regardless of the str type hint. Option B is wrong because a str annotation does not make routing greedy, and option A incorrectly confuses a router directive with Pydantic validation.

    Read the full bite: What is the :path converter in FastAPI?

  24. Question 24 of 30

    For which purpose are FastAPI query parameters most appropriately used?

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    Answer: b · To provide optional criteria for filtering or paginating a list of items.

    The card states query parameters are for optional data like filtering, pagination, or sorting a collection of resources. Options A and D describe the purpose of path parameters, while option A describes when to use a request body, not query parameters.

    Read the full bite: FastAPI Query Parameters: Beyond the URL Path

  25. Question 25 of 30

    Which method correctly enables automatic JSON body validation in a FastAPI route?

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    Answer: a · Subclass BaseModel and declare it as the type of a path operation function parameter

    FastAPI inspects path operation parameter type annotations to automatically parse and validate incoming JSON against a Pydantic BaseModel. Manually calling request.json() bypasses this automatic pipeline, and response_model only defines the outgoing response schema rather than request validation.

    Read the full bite: How do you define a Pydantic model for FastAPI request body validation?

  26. Question 26 of 30

    When a FastAPI endpoint receives JSON with extra fields not defined in the Pydantic model, what occurs by default?

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    Answer: d · Pydantic silently drops the extra fields and the request succeeds

    By default Pydantic ignores extra fields, silently dropping them so the model instantiates and the request succeeds. Option C is wrong because that strict 422 behavior only happens when you explicitly configure extra to forbid in model_config.

    Read the full bite: How does Pydantic handle extra JSON fields, and how to configure it?

  27. Question 27 of 30

    What is the primary benefit of using Uvicorn workers for a FastAPI application in production?

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    Answer: d · It allows the application to utilize multiple CPU cores for concurrent request processing.

    Uvicorn workers are designed to scale your application by running multiple processes, each capable of handling requests, thereby utilizing all available CPU cores to process requests concurrently. Options A and B are incorrect because workers actually complicate debugging and disable auto-reloading; option A is wrong as workers do not fix fundamentally slow or blocking application logic.

    Read the full bite: Uvicorn Workers: Scaling Your FastAPI App

  28. Question 28 of 30

    When developing a FastAPI application, for which purpose is a Pydantic BaseModel most effectively utilized?

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    Answer: d · To define the expected structure and types of data within a POST or PUT request's body.

    The card states that Pydantic models are used for 'the request body of any POST, PUT, or PATCH endpoint where the client sends structured data.' Options A, B, and D describe scenarios where Pydantic models are explicitly advised not to be used for the primary request body model, as those are handled by other FastAPI mechanisms like path parameters, query parameters, or Form().

    Read the full bite: FastAPI: Pydantic for Robust Request Bodies

  29. Question 29 of 30

    What is the key difference between a Pydantic field defined as name: str = 'guest' and one defined as name: Optional[str] = None?

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    Answer: d · The first rejects None while the second accepts it, but both may be omitted from input.

    Both fields have defaults so neither is required, yet str = 'guest' rejects None while Optional[str] = None accepts it. Distractor A is tempting because Optional sounds optional, but requiredness is determined solely by the presence or absence of a default.

    Read the full bite: What is the difference between a Pydantic default and Optional field?

  30. Question 30 of 30

    In Pydantic V2, how should you enforce a positive price and a regex-formatted SKU without writing custom validators?

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    Answer: b · Set price: float = Field(gt=0) and sku: str = Field(pattern=r'^ITEM-\d{5}$') on standard types

    Field's built-in gt and pattern parameters enforce constraints natively without extra code, while @field_validator adds unnecessary boilerplate and ignores Pydantic V2's native capabilities.

    Read the full bite: Enforce positive price and SKU format using Pydantic Field without custom validators

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