400 Python FastAPI Interview Questions with Answers 2026
Course Overview
What You'll Learn
- Core Fundamentals: Pydantic V2 models, Path vs. Query parameters, and Starlette/Uvicorn internals.
- Advanced Dependency Injection: Sub-dependencies, reusable trees, and stateful middleware.
- Database & Concurrency: async/await patterns, connection pooling, and Alembic migrations.
- Security & Auth: OAuth2 Password flow, RBAC (Role-Based Access Control), and JWT implementation.
- Testing & Observability: Testing with TestClient, Prometheus monitoring, and OpenAPI customization.
About This Free Course
Python FastAPI Interview & learn hashicorp terraform associate certification practice exams
Master FastAPI: High-Performance Web APIs with Python
Python FastAPI Interview and Certification Practice Questions are meticulously designed for developers and engineers who want to bridge the gap between basic syntax and production-grade API mastery. This comprehensive course targets the most sought-after skills in the modern Python ecosystem, ensuring you can confidently navigate everything from Pydantic V2 data validation and OAuth2/JWT security to complex asynchronous dependency injection and database scaling with SQLAlchemy and Alembic. Whether you are preparing for a senior-level technical interview or aiming to architect scalable microservices, these practice exams provide deep-dive explanations that clarify the "why" behind the code, covering critical topics like ASGI architecture, Pytest/HTTPX integration, and Redis-based caching to ensure your APIs are not just functional, but lightning-fast and secure.
Exam Domains & Sample Topics
Core Fundamentals: Pydantic V2 models, Path vs. Query parameters, and Starlette/Uvicorn internals.
Advanced learn dependency injection principles patterns architecture: Sub-dependencies, reusable trees, and stateful middleware.
Database & Concurrency: async/await patterns, connection pooling, and Alembic migrations.
Security & Auth: OAuth2 Password flow, RBAC (Role-Based Access Control), and JWT implementation.
Testing & Observability: Testing with TestClient, Prometheus monitoring, and OpenAPI customization.
1. When defining a path operation that requires an optional query parameter limit with a default value of 10 and a maximum constraint of 100, which approach is considered the "FastAPI way" using Pydantic integration?
A) def get_items(limit: int = 10)
B) def get_items(limit: int = Query(10, le=100))
C) def get_items(limit: Annotated[int, Query(10, gt=100)])
D) def get_items(limit: Annotated[int, Query(gt=0, le=100)] = 10)
E) def get_items(limit: int = Path(10, max_length=100))
F) def get_items(limit: int = Body(10, le=100))
Correct Answer: D
Overall Explanation: FastAPI uses the Query class (often wrapped in Annotated for modern Python) to add metadata and validation logic to query parameters. The le (less than or equal to) argument enforces the maximum value, while Annotated keeps the type hint clean and reusable.
Option A is incorrect: It provides a default value but lacks the "maximum 100" validation constraint.
Option B is incorrect: While functional, using Annotated (as seen in D) is the current best practice for PEP 593 compatibility and better IDE support.
Option C is incorrect: It uses gt (greater than) 100, which is the opposite of the "maximum 100" requirement.
Option D is correct: It correctly uses Annotated, sets a default of 10, and ensures the value is between 1 and 100.
Option E is incorrect: Path is used for path parameters (e.g., /items/{id}), not query parameters.
Option F is incorrect: Body is used for data sent in the request body (JSON), not as a URL query string.
2. In an asynchronous FastAPI endpoint, what happens if you perform a long-running, CPU-bound calculation using a standard def function without async?
A) FastAPI automatically runs it in a separate threadpool.
B) The entire event loop is blocked until the calculation finishes.
C) The request is immediately terminated with a 500 error.
D) It runs faster than an async def function due to lower overhead.
E) FastAPI converts the function to a coroutine at runtime.
F) The calculation is offloaded to a Background Task automatically.
Correct Answer: A
Overall Explanation: One of FastAPI's smartest features is how it handles def vs async def. When you define an endpoint with def, FastAPI assumes it might be blocking and runs it in an external threadpool (using anyio) to avoid freezing the main event loop.
Option A is correct: FastAPI detects the non-async signature and executes it in a threadpool so other requests can still be processed.
Option B is incorrect: This would only happen if you performed blocking I/O inside an async def function.
Option C is incorrect: This is valid Python/FastAPI syntax; no error is triggered.
Option D is incorrect: CPU-bound tasks are restricted by the GIL; "faster" is subjective and usually false here.
Option E is incorrect: FastAPI does not rewrite your Python code or change its type.
Option F is incorrect: BackgroundTasks must be explicitly declared and called by the developer.
3. Which component of the FastAPI security system is responsible for verifying the "scopes" required for a specific endpoint when using OAuth2?
A) HTTPBasic
B) OAuth2PasswordRequestForm
C) SecurityScopes
D) JOSE
E) APIKeyHeader
F) CORSMiddleware
Correct Answer: C
Overall Explanation: For fine-grained access control (RBAC), FastAPI provides the SecurityScopes class. When injected into a dependency, it allows the system to check if the token provided by the user contains the specific permissions (scopes) required by the route.
Option A is incorrect: HTTPBasic is for simple Username/Password headers, not OAuth2 scopes.
Option B is incorrect: This is a convenience class used to parse the username and password during the login/token-generation phase.
Option C is correct: SecurityScopes is the specific tool used within dependencies to enforce scope-based authorization.
Option D is incorrect: JOSE is a library used to sign/verify tokens, but it doesn't handle FastAPI dependency injection logic.
Option E is incorrect: This is used for simple API Key validation, which does not inherently support OAuth2 scopes.
Option F is incorrect: CORSMiddleware handles cross-origin requests, not user permissions.
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