Use this guide to interview a Python Developer for backend services, tooling, or data-adjacent work. Pick eight to twelve questions per session, keep the set consistent across candidates, and favor questions that connect language knowledge to shipped work — reciting trivia about Python internals matters far less than knowing when those internals bite. Anchor scenario questions in {{Your product or domain}}.

Python language depth

• Explain how the GIL affects concurrency, and how you work around it for CPU-bound versus I/O-bound workloads.
• When do you use asyncio versus threads versus multiprocessing? Give a real case for each.
• What are generators, and when have they materially improved a program you wrote?
• Explain decorators and describe a non-trivial one you have written.
• How do mutable default arguments bite people, and what other Python gotchas do you warn juniors about?
• What has changed in recent Python versions that you actually use — pattern matching, typing improvements, performance work?

What good looks like: strong candidates connect language features to real decisions rather than reciting textbook definitions — a generator that fixed a memory problem, a decorator that removed boilerplate across a codebase. They describe the GIL accurately, including what it does and does not serialize, instead of repeating folklore.

Frameworks and APIs

• Compare two Python web frameworks you have used — for example Django, FastAPI, or Flask. What drives your choice for a new service?
• How do you structure a Django or FastAPI project so it stays maintainable past fifty thousand lines?
• How do you handle background work — Celery, a task queue, or something else? What failure modes have you hit?
• Walk me through designing input validation and serialization for an API endpoint. Where do tools like Pydantic fit?
• ORM versus raw SQL: how do you decide, and where has the ORM hurt you?

Data handling and performance

• A Python service is slow. Walk me through profiling it — which tools, in what order?
• When does pandas stop being the right tool, and what do you reach for next?
• How do you process a file that is too large to fit in memory?
• Tell me about the biggest performance win you achieved in Python. What was the real bottleneck?
• When do you drop into C extensions, Rust, or vectorized numpy instead of optimizing pure Python?

Testing and code quality

• Describe your testing stack — pytest fixtures, mocking, property-based tests. What do you mock, and what do you refuse to mock?
• How do you use type hints and a checker like mypy? Where is strict typing worth it, and where does it fight you?
• What does a good code review comment from you look like? Give a real example.
• How do you test code that talks to external services?
• What linting and formatting do you enforce in CI, and which arguments do they end?

What good looks like: the best candidates treat typing and tests as design tools, not chores. They can explain exactly where they draw the mocking boundary and why over-mocked tests rot, and they describe types as documentation that catches real bugs at review time rather than ceremony imposed by tooling.

Packaging, environments, and deployment

• Walk me through your dependency management — pip-tools, Poetry, uv, lockfiles. How do you keep environments reproducible?
• How do you structure and publish an internal Python package that other teams depend on?
• What does your container image for a Python service look like, and how do you keep it small and fast to build?
• How do you handle configuration and secrets across environments?

Debugging and collaboration

• A production Python process is slowly eating memory. How do you find the cause?
• Tell me about a mysterious bug whose fix taught you something about Python itself.
• How do you upgrade a large codebase across a major dependency or Python version?
• How do you raise code quality in a legacy codebase without stopping feature work?
• Describe working with data scientists or other specialists — how do you get research code production-ready without friction?