hashable

Hashable is a term used in programming to describe objects that can be hashed to an integer and used as keys in hash-based data structures such as dictionaries and sets. For an object to be hashable, its hash value must be computed consistently from the object’s contents and must not change during the object’s lifetime. Additionally, the hash function should be compatible with the language’s notion of equality: if a == b, then hash(a) == hash(b). Collisions, where different objects share the same hash, are allowed but must be handled by the data structure, typically through comparing the objects themselves after a hash match.

In Python, hashability is implemented via the __hash__ and __eq__ methods. Most built-in immutable types (int, float, str, and tuples of hashable elements) are hashable. Mutable types such as lists or dicts are unhashable. If you override __eq__ in a class and do not define __hash__, the class becomes unhashable in Python 3. To create hashable custom types, implement __eq__ and __hash__ consistently, and ideally make instances immutable or effectively immutable.

Other languages provide similar concepts. In Java, objects used as keys must have reliable hashCode and equals implementations. In Rust, types implementing the Eq and Hash traits can be used as keys in hash maps. In C++, unordered_map requires a hash function specialization for the key type.

Hashable objects are central to the performance of hash tables, enabling average-case O(1) lookups. Designers often derive hash codes from immutable fields to preserve stability. Note that hash values are not guaranteed to be unique; multiple objects may share a hash value.