Unhashable
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.
Python How-To
An unhashable object is a Python object that cannot serve as a dictionary key or set element because it lacks the required stable hashability.
- How it is used
- Explains why dictionaries and sets reject mutable objects and how to choose containers for retrieval or membership operations.
- How deep it goes
- Explains TypeError, stable hash values, mutability conflicts, common types, and runtime checking without descending into hashing algorithms.
- Examples
- Uses failed dictionary and set constructions, hash calls, a hashability-checking loop, and comparisons of mutable and immutable types.
- What you can do after
- Diagnose TypeError from invalid dictionary keys or set elements and check candidate values with collections.abc.Hashable.
Python Workout: 50 ten-minute exercises
Unhashable describes a Python object whose value cannot be used as a dictionary key or set element because it lacks a suitable stable hash value.
- How it is used
- Explains why dictionaries and sets require objects whose hash remains stable.
- How deep it goes
- Explains mutation, changing hashes, and unreachable key-value pairs while deferring finer distinctions between hashability and immutability.
- Examples
- Uses mutable lists as dictionary keys to demonstrate why changing a key could make its stored value unfindable.
- What you can do after
- Diagnose why a mutable object cannot be used as a dictionary key or set element.
Read how Python Workout: 50 ten-minute exercises explains unhashable →
Related concepts
- Mapcontrasts with · A map is an associative data structure that stores key-value pairs and supports retrieving values by key.
- setcontrasts with · A set is a collection of unique elements in which membership is determined by whether an element is present, not by duplicate occurrences.
- hashablecontrasts with · A hashable object has a stable hash value and compatible equality behavior, allowing it to be used as a dictionary key or set element.
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