What is the __hash__ and __eq__ contract? | Interview Question

Q: What is the __hash__ and __eq__ contract?

**Target Audience:** Developers who have hit `TypeError: unhashable type` or want their own class as a dict key, up to candidates who need to explain how dicts use the two methods. **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, which is CPython 3.11.3); Example 2 was also run on 3.13 with the same output. > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First A big coat check has a thousand numbered hooks. When you hand in your coat, the attendant does not search all thousand hooks; they look at the colour of your ticket, which tells them which row of ten hooks to check, and then compare tickets only within that row. That only works if **the same c

The problem

Question presented to candidate: "If I write a class and want to use its instances as dictionary keys or put them in a set, what do __eq__ and __hash__ have to do, and what goes wrong if they disagree?"

What a strong answer should cover:

The contract: if a == b then hash(a) == hash(b); the reverse is not required (collisions are allowed). A hash must also stay the same for the object's whole life while it is in a set or dict.
Defaults: plain objects compare and hash by identity. Defining __eq__ without __hash__ makes Python 3 set __hash__ = None, so instances become unhashable (TypeError: unhashable type).
How dicts and sets use it: hash picks the slot, then identity is checked, then __eq__; breaking the contract gives silent duplicates or lookups that miss.
Implement both from the same immutable fields: __eq__ compares a tuple of fields (returning NotImplemented for other types), __hash__ returns hash() of the same tuple; mutable objects should not be hashable.
Tools and details: @dataclass(frozen=True) generates both, eq=True alone removes the hash, unsafe_hash=True is risky; 1 == 1.0 == True share a hash; string hashes are randomised per process (default since 3.3).

Clarifying questions expected:

Will the objects be mutated after they are added to a set or used as keys?

Code / implementation expected: Yes — a value class with consistent __eq__/__hash__, and a demonstration of what breaks when the contract is violated or a key is mutated.

hashing
eq

Try it yourself

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Solution

Target Audience: Developers who have hit TypeError: unhashable type or want their own class as a dict key, up to candidates who need to explain how dicts use the two methods. Difficulty: Medium Python version: Behaviour checked on CPython 3.11 (python3.11 and the in-page Pyodide 0.25.1 runtime, which is CPython 3.11.3); Example 2 was also run on 3.13 with the same output.

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. Every output in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

A big coat check has a thousand numbered hooks. When you hand in your coat, the attendant does not search all thousand hooks; they look at the colour of your ticket, which tells them which row of

The solution is hidden so you can try first

13 min read · 12 sections

Run the code

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

PythonThe contract, the __eq__-only trap, broken hashes and mutated keys

Practice

PythonHow dicts call __eq__, NaN, dataclasses and subclasses

Practice

Discussion

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What is the hash and eq contract?