Python in a Nutshell, 4th Edition

Chapter 7. Modules and Packages

A typical Python program is made up of several source files. Each source file is a module, grouping code and data for reuse. Modules are normally independent of each other, so that other programs can reuse the specific modules they need. Sometimes, to manage complexity, developers group together related modules into a package—a hierarchical, tree-like structure of related modules and subpackages.

A module explicitly establishes dependencies upon other modules by using import or from statements. In some programming languages, global variables provide a hidden conduit for coupling between modules. In Python, global variables are not global to all modules, but rather are attributes of a single module object. Thus, Python modules always communicate in explicit and maintainable ways, clarifying the couplings between them by making them explicit.

Python also supports extension modules—modules coded in other languages such as C, C++, Java, C#, or Rust. For the Python code importing a module, it does not matter whether the module is pure Python or an extension. You can always start by coding a module in Python. Should you need more speed later, you can refactor and recode some parts of your module in lower-level languages, without changing the client code that uses the module. Chapter 25 (available online) shows how to write extensions in C and Cython.

This chapter discusses module creation and loading. It also covers grouping modules into packages, using ...