Manufacturing the Missing Bridge: A Scaffolding Design for Function Introduction in a Python-First K-12 Informatics Curriculum

Scratch-then-Python pedagogies leverage the My Blocks feature of Scratch as an informal bridge to procedural abstraction: by the time a learner meets Python's def, the intent of name a reusable parameterized action is already familiar and only the syntax is new. Python-first curricula do not enjoy this bridge. We diagnose the function-introduction bottleneck in a Pythonfirst K-12 cohort across four canonical dimensions (abstraction intent, return-value semantics, parameter passing, lexical scope), rank the dimensions by severity against the Python-first profile, and propose a two-chapter scaffolding sequence at the Volume 1 → Volume 2 boundary of the Python Pathway curriculum. The proposed sequence combines three levers established in the computing-education literature — subgoal labeling on worked examples (Margulieux et al. 2012, 2016, 2019), parametertracing notional-machine diagrams (Sorva 2013), and functionas-plan framing (Soloway 1986) — with an explicit deferral of scope and multi-value return until Volume 2. The design is evaluated analytically against the Lopez-Whalley tracingexplaining-writing hierarchy and the McGill-Volet abstractionintent construct, with empirical validation flagged as future work under the program's no-cohort scope.