Hashable
A hashable object has a stable hash value and compatible equality behavior, allowing it to be used as a dictionary key or set element.
Python How-To
A hashable object is a Python object with a stable hash value and equality behavior that allow it to serve as a dictionary key or set element.
- How it is used
- Guides the choice of objects used as dictionary keys and set elements.
- How deep it goes
- Explains hash consistency, collisions, mutability, common types, and runtime checks, but does not detail hash-table implementation protocols.
- Examples
- Uses failed dictionary and set constructions, hash calls, and a Hashable-based type-checking script.
- What you can do after
- Diagnose TypeError failures and select suitable dictionary keys or set elements using Hashable checks.
Python Workout: 50 ten-minute exercises
A hashable type has values with stable hash values and can therefore be used for dictionary keys or set elements.
- How it is used
- Explains why dictionary keys must be hashable and how that requirement supports unique, efficient lookup.
- How deep it goes
- Defines hashability through hash(), contrasts O(n) sequence search with O(1) dictionary lookup, and only raises collision handling.
- Examples
- Uses an office-directory analogy and contrasts strings, lists, and tuples with dictionary lookup.
- What you can do after
- Understand which values can serve as dictionary keys and why hashing makes lookup efficient.
Read how Python Workout: 50 ten-minute exercises explains hashable →
Python Workout, Second Edition
A hashable object is a Python object with a stable hash value that can be used as a dictionary key.
- How it is used
- Uses hashability to determine which Python objects can act as dictionary keys and support efficient lookup.
- How deep it goes
- Explains hash functions through lookup behavior and complexity, but stops short of implementation details.
- Examples
- Uses numbers and strings as keys and contrasts unrestricted dictionary values with unique keys.
- What you can do after
- Apply it by selecting hashable objects for dictionary keys while storing arbitrary values.
Related concepts
- Mapused by · A map is an associative data structure that stores key-value pairs and supports retrieving values by key.
- unhashablecontrasts with · In Python, unhashable describes an object that cannot be used as a dictionary key or set element because it lacks a suitable stable hash value.
- statecontrasts with · State is information or a configuration describing the condition of a system, entity, or computation at a particular time and potentially influencing subsequent behavior or decisions.
- immutable collectionequivalent to · An immutable collection is a collection whose membership and contents cannot be changed after creation.
- similarity searchproduces · Similarity search is a retrieval technique that finds or ranks items by comparing their representations with a query according to a similarity or distance measure.
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