Build It in Python

Move from writing scripts to structuring applications. Programs start to take shape around classes that hold both data and behavior. Functions become methods. Data that was loosely stored earlier is replaced with objects that define what belongs and what does not. Work moves through a sequence of projects that grow in scope. A contact book turns into a multi-class program with persistence. An expense tracker adds filtering, summaries, and validation. A task manager expands into a multi-file package with a command line interface and tests. Each step builds on what came before. Existing code is reorganized rather than replaced. Small pieces are extended and combined until they form complete programs. Object structure, inheritance, method design, special methods, and comprehensions are introduced where they are needed. Classes begin to behave like built-in types, so sorting, comparison, and membership work directly on your objects. Setup stays minimal. The focus remains on writing and organizing code so programs stay readable as they grow. Intended for readers who can already write basic programs and want to turn them into structured applications. Part of the Build It in Python series, between first projects and production-level systems.