Subconstruct

Subconstruct is a term used in the Construct library, a declarative framework for defining and parsing binary data structures in Python. In this context, a subconstruct refers to a component that resides inside a larger construct and describes how a portion of the input should be parsed or built. A construct may contain one or more subconstructs, each representing a field or a nested structure.

Conceptually, a subconstruct wraps another construct (often called the subcon) and delegates the actual parsing and building work to that inner component. The wrapper may also add behavior such as naming, labeling, context propagation, or modification of the parsing state, while the inner subcon remains responsible for the core data interpretation. This separation enables modular composition, where complex data layouts are assembled from smaller, reusable parts.

Subconstructs play a key role in enabling wrappers and adapters within Construct. They allow users to apply operations like renaming fields, applying optionality, counting repeated elements, or selecting a variant with a Switch, without changing the underlying data description. By stacking subconstructs, developers can build rich, nested structures that are easy to read and maintain.

In practice, high-level constructs such as Struct, Array, Enum, and If-Then-Else are composed from subconstructs. While end users typically work with these higher-level abstractions, the subconstruct mechanism provides the flexible foundation that makes complex, modular binary parsing possible.