What are dataclasses? | Interview Question

Q: What are dataclasses?

**Target Audience:** Python developers who write classes by hand and want to know what `@dataclass` generates, which options matter, and where its limits are. **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11.17, 3.12 and 3.13, and in the page runner (Pyodide 0.25.1, CPython 3.11.3). `slots`, `kw_only` and `KW_ONLY` need 3.10+. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First Imagine filling in the same paper form for every new customer: name in box one, email in box two, copy the boxes onto a receipt, then compare two forms box by box to see if they are the same customer. It is boring, and the day someone adds a "phone" box, three different places have to change and one of them is always

The problem

Question presented to candidate: "We have a lot of small classes that just hold data and every one has the same hand-written __init__, __repr__ and __eq__. What does @dataclass do for us, how do you configure it, and where does it fall short?"

What a strong answer should cover:

@dataclass (standard library since Python 3.7, PEP 557) reads the annotated class attributes as fields and generates __init__, __repr__ and __eq__ from them, in declaration order
Defaults and the mutable-default rule: list, dict and set defaults are rejected with ValueError; use field(default_factory=list)
__post_init__ for validation and derived fields, field(init=False), InitVar, and helpers asdict, replace, fields
Options: frozen=True (immutable and hashable), order=True (tuple-style comparisons), slots=True and kw_only=True (3.10+); eq=True alone makes instances unhashable
Limits: type hints are not validated at runtime (that is what Pydantic is for), default-before-non-default ordering breaks with inheritance, and how it compares with NamedTuple and plain classes

Clarifying questions expected:

Do the objects need to be immutable or usable as dict keys?
Is the input trusted, or does it need runtime validation (for example parsed JSON)?

Code / implementation expected: Yes — a small dataclass with a default_factory field and a __post_init__ check, ideally a frozen one used as a dict key.

dataclasses

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Solution

Target Audience: Python developers who write classes by hand and want to know what @dataclass generates, which options matter, and where its limits are. Difficulty: Medium Python version: Behaviour checked on CPython 3.11.17, 3.12 and 3.13, and in the page runner (Pyodide 0.25.1, CPython 3.11.3). slots, kw_only and KW_ONLY need 3.10+.

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. All output shown was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

Imagine filling in the same paper form for every new customer: name in box one, email in box two, copy the boxes onto a receipt, then compare two forms box by box to see if they are the same customer. It is boring, and

The solution is hidden so you can try first

11 min read · 11 sections

Run the code

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

PythonWhat @dataclass generates, default_factory, __post_init__ and the helpers

Practice

Pythonfrozen, order, slots, kw_only, InitVar, inheritance and match

Practice

Discussion

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