What makes an object hashable in Python, and why does it matter?

Connects stable hashing and equality to dictionary and set correctness, including custom types and mutable-key failures.

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TL;DR

An object is hashable when it has a hash value that remains stable during its lifetime and it can be compared for equality. Objects that compare equal must have the same hash. Dictionaries use hashable keys and sets store hashable members. Immutable built-ins such as strings and numbers are hashable; lists, dictionaries, and sets are not because their changing contents would make content-based hashing unstable.

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The hash/equality contract, built-in hashability rules, average lookup behavior, and safe value-object design.

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Core answer
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Overview
Hashability is a behavioral contract that lets dictionaries and sets find a value again after placing it in a bucket.
An object is hashable when it has a hash value that remains stable during its lifetime and it can be compared for equality. Objects that compare equal must have the same hash. Dictionaries use hashable keys and sets store hashable members. Immutable built-ins such as strings and numbers are hashable; lists, dictionaries, and sets are not because their changing contents would make content-based hashing unstable.
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A tuple is hashable only if every element is hashable. User-defined instances are often identity-hashable by default, but defining value equality changes the design: the hash must use the same stable fields or hashing should be disabled. A frozen dataclass may generate value-oriented hashing when its fields support it, yet frozen is only shallow—placing mutable nested state in the equality contract can still break the intended value semantics.

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Assuming a hash must be unique.
Changing equality fields while an object is stored as a key.
Believing every immutable-looking container is hashable.
Using a hash as authentication or as a durable identifier.

What makes an object hashable in Python, and why does it matter?

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