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Chapter 20: Modules and Functions

Chapter 20 teaches you how to organize and reuse Python code through modules and functions. Modules are .py files containing reusable code. Functions are reusable blocks of code with clear parameters and return values.

This chapter bridges foundational Python syntax (Chapters 13-19) with production-oriented patterns by teaching you to:

  • Describe intent through function signatures with type hints
  • Organize code into logical modules with clear separation of concerns
  • Reuse code through imports and function calls
  • Validate behavior through testing

By the end of this chapter, you'll build a real multi-module project (Calculator Utility) that demonstrates professional Python organization patterns.

What You'll Learn​

Lesson 1: Understanding Modules and Imports (A2 - 55 min)​

Learn what a module is and how Python organizes code. Explore three import patterns:

  • import module_name — Import entire module, access via dot notation
  • from module import function — Import specific function, use directly
  • from module import function as alias — Import with custom name

What you'll understand: Modules are tools Python provides. You can use built-in modules (math, random, os) immediately.

Lesson 2: Writing Functions with Intent (A2-B1 - 60 min)​

Learn to write functions with clear parameters, return values, type hints, and docstrings.

What you'll understand: Type hints and docstrings are your specification. They tell other developers (and AI) exactly what your function needs and produces.

Lesson 3: Function Parameters and Returns (B1 - 60 min)​

Master positional parameters, default parameters, keyword arguments, and returning multiple values.

What you'll understand: Functions have flexible calling patterns. You can design them to work in multiple ways while maintaining clarity.

Lesson 4: Scope and Nested Functions (B1-B2 - 55 min)​

Understand variable scope (local, global, enclosing) and how nested functions work with closures.

What you'll understand: Variables have regions where they exist. Understanding scope prevents bugs and clarifies your design.

Lesson 5: Building a Calculator Utility Capstone (B1-B2 - 70 min)​

Integrate all concepts by building a real multi-module calculator project.

What you'll understand: Professional Python code uses modules for separation of concerns, clear functions with type hints, proper testing, and clean orchestration.