unhashable

Unhashable is a term used in programming to describe objects that cannot be hashed, meaning they do not provide a valid, stable hash value for the lifetime of the object. Hashing assigns a numeric value to an object, enabling fast lookups in data structures such as dictionaries and sets. For an object to be hashable, it must implement a hash function and have a well-defined notion of equality that is consistent with that hash.

In Python, hashability is tied to immutability and the presence of a suitable hash method. Most built-in immutable types, such as integers, floats, strings, and tuples of hashable elements, are hashable. Tuples are hashable only if all their elements are hashable. Conversely, mutable containers such as lists, dictionaries, and sets are unhashable because their contents can change, which would alter their hash value and compromise lookup integrity. Attempting to use an unhashable object as a dictionary key or as a member of a set typically raises a TypeError like "unhashable type: 'list'."

Objects can become hashable or remain hashable in practice by design. For example, a class can define a __hash__ method and ensure its instances are immutable in effect. Dataclass or dataclass(frozen=True) can also influence hashability. In cases where mutability is unavoidable, developers often use a hashable proxy (such as a tuple or a frozenset) or use the object as a value rather than a key.

In summary, unhashable indicates that an object cannot participate in hashing-based collections, a constraint that affects how data structures like dictionaries and sets are used.