Subtemplates

Subtemplates are small template fragments intended for inclusion within larger templates. They enable modular design by isolating repeatable pieces such as headers, footers, sidebars, cards, or widgets. Subtemplates are stored separately and rendered as part of a parent template, often with a mechanism for passing data or parameters.

Most templating systems provide a way to incorporate subtemplates, such as include, import, or extend directives, or via macro or partial features. The parent template calls the subtemplate and the engine renders it with a local scope or provided data. Subtemplates can be used to compose complex pages from reusable building blocks. Examples across ecosystems include including a header subtemplate within every page, or using a product-card subtemplate within a catalog listing. In Django and Jinja, includes are common; Handlebars uses partials; PHP uses include or require; Smarty uses include.

Benefits include reuse, consistency, and easier maintenance. Subtemplates separate structure from content, support collaboration, and simplify caching and testing. Potential drawbacks include tight coupling between parent and subtemplate data, data scope issues, and the risk of over-nesting; care is needed to avoid circular references and to control data passed to subtemplates and escaping rules for security.

Best practices include defining stable, minimal interfaces for subtemplates, passing only what is needed, keeping logic out of templates, documenting dependencies, and choosing clear, descriptive names. When designing a template system, consider performance implications and whether to cache rendered subtemplates.