mutability

Mutability is the property of an object or data structure that allows its state or content to be changed after it is created. Objects that do not allow modification are described as immutable. Mutability is a core concept in programming languages, data modeling, and software design, influencing how data is stored, accessed, and updated.

In many programming languages, mutability applies to objects or collections. For example, Python lists, dictionaries, and sets are mutable, allowing in-place updates. Strings and tuples are immutable in Python. In Java, most objects are mutable by default, while strings are immutable. Locking, references, and aliasing can complicate mutability because multiple names can refer to the same underlying object.

Mutability has trade-offs. Mutable state can simplify algorithms that update data in place and can reduce memory allocations, but it also introduces potential side effects, making reasoning about code harder and increasing the risk of bugs, especially in concurrent environments. Immutable data structures avoid these issues by ensuring that operations yield new values rather than altering existing ones, often enabling easier reasoning and improved thread safety.

Design patterns often manage mutability through encapsulation, using private state and accessors or through functional techniques such as persistent data structures and structural sharing to simulate immutability while maintaining performance. In databases and systems, mutability is governed by transactions, versioning, and access controls to preserve consistency.