Python Developer Interview Questions UK: 8 Real Questions and What Interviewers Want

Before your Python Developer interview, it helps to see the questions from the interviewer's side. We have picked eight from our set of 25 and added the reason each one gets asked.

Eight Python Developer interview questions, and what the interviewer is thinking

1. What is the difference between a list and a tuple in Python, and when would you use each?

Why they ask: A staple Python question checking that you understand mutability and idiomatic use of core types.

2. Explain decorators in Python and give an example where you've used one.

Why they ask: Decorators are a core Python feature, and explaining them well shows understanding of functions as first-class objects.

3. What are generators and iterators in Python, and why would you use them?

Why they ask: Memory-efficient data processing is common in Python roles, and generators show practical understanding.

4. How does Python manage memory, and what is the Global Interpreter Lock?

Why they ask: Understanding the GIL is essential for choosing the right concurrency approach in Python.

5. When would you use asyncio rather than threads or multiprocessing?

Why they ask: Choosing the right concurrency model is a frequent mid-level Python question.

6. A Python script that processes a large CSV file keeps running out of memory. How would you fix it?

Why they ask: Handling large data efficiently is a very common real-world Python problem.

7. Tell me about a Python project or pipeline you built that you are proud of.

Why they ask: They want concrete evidence of what you have built, your technical choices and the impact.

8. Why do you want to be a Python developer, and what kind of Python work do you enjoy most?

Why they ask: They want to understand your motivation and whether your interests match the work on offer.

A sample answer to question 1

What is the difference between a list and a tuple in Python, and when would you use each?

Both are ordered sequences, but a list is mutable, so I can append, remove and modify items after creation, whereas a tuple is immutable. That difference drives how I use them. I'd use a list for a collection that changes or is homogeneous, like a series of order IDs I'm building up. I'd use a tuple for a fixed record of related values, such as a coordinate pair, or returning multiple values from a function. Because tuples are immutable and hashable, provided their contents are hashable, they can be used as dictionary keys or set members, which lists can't. They're also slightly smaller and faster to create. For fixed records with named fields, I'd prefer a namedtuple or, more often now, a dataclass, because point.x is clearer than indexing at position zero. One gotcha is that a tuple containing a mutable object, such as a list, can still have that inner object changed, so immutability is shallow. It's about signalling intent as much as performance.

Common mistake to avoid: Claiming tuples are always faster by a large margin; Forgetting shallow immutability

Want the rest?

Full model answers, key points and common mistakes for all 25 questions are on the Python Developer interview questions page. The first five are free, no sign-up needed.