Question 1

Is FastAPI better than Flask in 2026?

Accepted Answer

For new API services, almost always yes — FastAPI ships with validation, async support, OpenAPI docs, and dependency injection that you'd otherwise bolt onto Flask via flask-pydantic, flask-async, flask-restx, etc. Flask is still a good choice for non-API web apps (server-rendered HTML, simple admin tools) where these features aren't useful.

Question 2

How much does a FastAPI developer earn in India?

Accepted Answer

₹8-26 LPA in 2026 for mid-to-senior backend developers with FastAPI as their primary stack. Companies hiring: Razorpay, Swiggy, Zerodha, Postman, CRED, Cure.fit. Specialised areas (FinTech, ML APIs) pay at the upper end.

Question 3

What Python version should I use with FastAPI in 2026?

Accepted Answer

Python 3.12 is the current sweet spot — faster than 3.11, broad library support, type-syntax improvements (PEP 695). Python 3.13 (no-GIL experimental) is exciting for FastAPI specifically but still considered alpha for production.

Question 4

How does FastAPI compare to Node.js for backend APIs?

Accepted Answer

Similar throughput for I/O-bound APIs (both async, both event-loop based). FastAPI wins on type safety and request validation out of the box. Node.js wins on raw npm ecosystem size and faster cold-start in serverless. For typical SaaS APIs in 2026, the choice is mostly team preference.

Question 5

Do I need to learn Starlette before FastAPI?

Accepted Answer

No. FastAPI hides Starlette completely for most use cases. You only need to drop down to Starlette for unusual middleware, custom ASGI handling, or low-level WebSocket / SSE work — and even then, FastAPI docs walk you through it.

Question 6

What is FastAPI and what problems does it solve?

Accepted Answer

FastAPI is a modern Python web framework for building APIs, released in 2018 by Sebastián Ramírez. It solves three problems that older Python frameworks (Flask, Django REST) handled poorly: (1) request validation requires manual code or external libraries, (2) async support is bolted-on rather than native, and (3) API documentation has to be written separately from the code. FastAPI handles all three automatically: Python type hints become Pydantic validation at runtime, every endpoint is async-capable, and OpenAPI/Swagger docs are generated from the function signature. The framework is built on Starlette (ASGI) and Pydantic, which means it gets concurrency from the former and type safety from the latter.

Question 7

How do you define a simple FastAPI endpoint with path and query parameters?

Accepted Answer

Path parameters are declared by including them in the route decorator and as function arguments with type annotations. Query parameters are any function arguments not in the path. FastAPI parses URL components and converts them to the annotated types — invalid input returns a 422 automatically.

Question 8

What is Pydantic and why does FastAPI use it?

Accepted Answer

Pydantic is a Python library for data validation using type hints, originally built by the same author as FastAPI. FastAPI uses Pydantic to validate request bodies, query parameters, and response models. When you declare a function parameter as a Pydantic model, FastAPI: parses the incoming JSON, validates each field against the model's type annotations, returns a structured 422 error if validation fails, and passes a fully-typed model instance to your function. Pydantic v2 (2023) rewrote the core in Rust, making it 5-50× faster than v1. This is the layer that lets FastAPI feel both 'magical' and safe.

Question 9

How do you return a typed response model in FastAPI?

Accepted Answer

Use the `response_model` parameter on the route decorator. FastAPI will (1) validate your return value against the model, (2) coerce/strip extra fields, (3) generate the response schema in the OpenAPI docs. This is crucial for hiding internal fields (passwords, internal IDs) from API responses.

Question 10

How do you handle file uploads in FastAPI?

Accepted Answer

Use `File()` or `UploadFile` as a parameter type. `UploadFile` is preferred for large files because it streams to disk instead of loading into memory. FastAPI parses `multipart/form-data` automatically.

Question 11

What's the difference between `def` and `async def` in FastAPI route handlers?

Accepted Answer

FastAPI supports both. A `def` (sync) handler runs in a threadpool — useful for blocking code like synchronous database drivers (psycopg2). An `async def` handler runs in the main event loop — required for `await`ing async code (httpx, asyncpg, SQLAlchemy async). The rule: if your handler awaits anything, it must be async; if it does CPU-bound work, sync is fine. Mixing is the most common FastAPI bug — calling a blocking function inside an `async def` blocks the whole event loop.

Question 12

How do you read request headers in FastAPI?

Accepted Answer

Use the `Header()` dependency. FastAPI converts `-` to `_` in header names by default (so `X-Token` becomes the parameter `x_token`). You can disable this with `convert_underscores=False`.

Question 13

What is dependency injection in FastAPI?

Accepted Answer

FastAPI's `Depends()` system lets you declare reusable functions whose return values are injected into route handlers. Common uses: shared database connections, auth checks, query parameter parsing, feature flags. The dependency is itself a function — FastAPI calls it, awaits its result, and passes it in. Dependencies can have dependencies, forming a tree FastAPI resolves per request.

Question 14

How do you set status codes and custom response headers?

Accepted Answer

Use the `status_code` parameter on the route decorator for the default status. For dynamic status or headers, accept `Response` as a parameter and mutate it. For one-off responses, return a `JSONResponse` / `Response` object directly with the status and headers you want.

Question 15

How does FastAPI generate OpenAPI/Swagger docs automatically?

Accepted Answer

FastAPI introspects every route handler at startup: it reads the type hints, response_model, path/query/body params, status codes, and docstrings to build a complete OpenAPI 3.1 spec. The spec is served at `/openapi.json` and rendered as Swagger UI at `/docs` and ReDoc at `/redoc`. You can add tags, summaries, and examples via decorator arguments. For production, you can disable the docs (`FastAPI(docs_url=None)`) or protect them behind auth.

FastAPI Interview Questions

What is FastAPI and what problems does it solve?

How do you define a simple FastAPI endpoint with path and query parameters?

What is Pydantic and why does FastAPI use it?

How do you return a typed response model in FastAPI?

How do you handle file uploads in FastAPI?

What's the difference between `def` and `async def` in FastAPI route handlers?

How do you read request headers in FastAPI?

What is dependency injection in FastAPI?

How do you set status codes and custom response headers?

How does FastAPI generate OpenAPI/Swagger docs automatically?

How do you serve static files in FastAPI?

What is CORS and how do you enable it in FastAPI?

How do you implement JWT authentication in FastAPI?

How do you connect FastAPI to a SQL database?

What are background tasks and when should you use them?

How do you handle exceptions globally in FastAPI?

What is the difference between `Depends()` and `Annotated[X, Depends()]`?

How do you write tests for FastAPI endpoints?

How do you implement rate limiting in FastAPI?

What is middleware in FastAPI and how is it different from dependencies?

How do you stream large responses in FastAPI?

How do you handle WebSockets in FastAPI?

What's the difference between query parameters, path parameters, and request body?

How do you validate complex query parameter combinations?

How do you optimize FastAPI for high-throughput production workloads?

How do you implement OAuth2 with multiple providers (Google, GitHub) in FastAPI?

How would you architect a FastAPI microservice for 10,000+ requests per second?

How do you implement server-sent events (SSE) for real-time updates?

How do you handle multi-tenancy in a FastAPI application?

Companies Hiring FastAPI

Salary Insights

Frequently Asked Questions

Is FastAPI better than Flask in 2026?

How much does a FastAPI developer earn in India?

What Python version should I use with FastAPI in 2026?

How does FastAPI compare to Node.js for backend APIs?

Do I need to learn Starlette before FastAPI?

Introduction