15.1 Why Use Modules?

Modules provide an easy way to organize components into a system, by serving as packages of names. From an abstract perspective, modules have at least three roles:

Code reuse

As we saw in Chapter 3, modules let us save code in files permanently. Unlike code you type at the Python interactive prompt, which goes away when you exit Python, code in module files is persistent—it can be reloaded and rerun as many times as needed. More to the point, modules are a place to define names, or attributes, that may be referenced by external clients.

System namespace partitioning

Modules are also the highest-level program organization unit in Python. Fundamentally, they are just packages of names. Modules seal up names into self-contained packages that avoid name clashes—you can never see a name in another file, unless you explicitly import it. In fact, everything "lives" in a module: code you execute and objects you create are always implicitly enclosed by a module. Because of that, modules are a natural tool for grouping system components.

Implementing shared services or data

From a functional perspective, modules also come in handy for implementing components that are shared across a system, and hence only require a single copy. For instance, if you need to provide a global object that's used by more than one function or file, you can code it in a module that's imported by many clients.

To truly understand the role of modules in a Python system, though, we need to digress for a moment and explore the general structure of a Python program.