# Composite ## Intent Treat individual objects and compositions uniformly through a part-whole tree. ## Problem and forces Clients must perform the same operation on leaves and nested groups. ## Applicability Use when the domain is genuinely hierarchical and uniform operations have coherent semantics. ## When not to use Avoid when leaf and container behavior differs materially or arbitrary nesting is unsafe. ## Benefits Simplifies tree traversal and recursive composition. ## Liabilities Can weaken type distinctions and make constraints on valid children difficult. ## Implementation considerations Define ownership, cycle prevention, traversal cost, mutation, and partial-failure behavior. ## Python example **Example context:** A file browser calculates storage size uniformly for individual files and folders containing nested items. ```python from dataclasses import dataclass from typing import Protocol class Node(Protocol): def size(self) -> int: ... @dataclass(frozen=True) class File: bytes: int def size(self) -> int: return self.bytes @dataclass(frozen=True) class Folder: children: tuple[Node, ...] def size(self) -> int: return sum(child.size() for child in self.children) ``` Leaves and composites share the `Node` contract, so clients calculate size uniformly across the tree. ## Credible alternatives Explicit tree nodes, Visitor, flat collections, or domain-specific aggregates. ## Related patterns Decorator, Iterator, Visitor. ## Architecture interview questions Which operations are truly uniform, and what child combinations are invalid?