What are list/dict/set comprehensions? | Interview Question

Q: What are list/dict/set comprehensions?

**Target Audience:** Beginners to intermediate Python developers — you should be comfortable with `for` loops and lists. **Difficulty:** Easy **Python version:** Behaviour checked on CPython 3.11, with the scope and bytecode differences also run on 3.12 and 3.13; the examples run in the page on Pyodide 0.25.1 (CPython 3.11.3). > **How to read this doc:** Every idea is explained in plain language first, then the precise word an interviewer expects is tagged `📌 Interview term:` and defined. The Quick Glossary at the end collects them. Output and bytecode shown come from real runs. ## 1. Why This Even Matters — A Story First You ask a friend at a market: "Get me the ripe ones from this crate, peeled." There are two ways to describe that job. The step-by-step way: "Take an empty bag. Pick up each fruit. If it is ripe, peel it and put it in the bag. When the crate is empty, give me the bag." The declarative way: "The peeled versions of the ripe fruits in this crate." The second sentenc

The problem

Question presented to candidate: "What are list, dict and set comprehensions in Python? Write a couple, tell me how they relate to a normal for loop, and when you would not use one."

What a strong answer should cover:

The shape [expression for item in iterable if condition]; [] builds a list, {} with key: value a dict, {} with single values a set; () gives a lazy generator expression, not a "tuple comprehension".
A trailing if filters items; x if cond else y in front transforms every item — different positions, different jobs.
Several for clauses read left to right like nested loops; a comprehension inside a comprehension builds a nested result.
Scope: the loop variable does not leak, but a walrus := binds in the enclosing scope; in a class body only the first iterable sees class names.
CPython detail: up to 3.11 a comprehension runs as a hidden nested function (<listcomp>); PEP 709 inlined list, dict and set comprehensions in 3.12.
When not to: side effects, deeply nested or multi-condition logic, or huge data where a generator expression avoids building the whole list.

Code / implementation expected: Yes — one list, one dict and one set comprehension, plus the equivalent loop for one of them.

comprehension

Try it yourself

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A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners to intermediate Python developers — you should be comfortable with for loops and lists. Difficulty: Easy Python version: Behaviour checked on CPython 3.11, with the scope and bytecode differences also run on 3.12 and 3.13; the examples run in the page on Pyodide 0.25.1 (CPython 3.11.3).

How to read this doc: Every idea is explained in plain language first, then the precise word an interviewer expects is tagged 📌 Interview term: and defined. The Quick Glossary at the end collects them. Output and bytecode shown come from real runs.

1. Why This Even Matters — A Story First

You ask a friend at a market: "Get me the ripe ones from this crate, peeled." There are two ways to describe that job. The step-by-step way: "Take an empty bag. Pick up each fruit. If it is ripe, peel it and put it in the bag. When the crate is empty, give me the ba

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10 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonList, set, dict and generator comprehensions side by side

Practice

PythonScope rules: no leaking, walrus, class bodies, eager vs lazy

Practice

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