Let's consider the role of Django apps in our e-commerce platform. The "solving small problems approach" fits well; many pieces of our e-commerce project will be common across multiple sites. By keeping our apps small and focused, we will be able to assemble the individual components in different ways for different projects.

For example, two e-commerce stores may share the same payment processor, but have entirely different needs for interacting with their customers. One site might need the ability to send an e-mail newsletter, while the other would not. If we were to build our Django project in large, monolithic sections, it would require more time and effort to satisfy the needs of these two projects. If, however, we use small, tiny pieces, we can simply plug-in the parts they have in common and upgrade or build separately the pieces that differ.

In larger development shops, this also allows for the internal reuse of apps across departments or functional groups. It can make code sharing and reuse much easier for in-house teams.

Keeping a modular design has other advantages. When a project decides to change payment processors, the upgrade is much simpler when the processing code lives alone in its own module. We can standardize the interface across all payment processors so other apps can interact with all of them the same way. In Python this is sometimes called "duck typing" and we will explore it more in Chapter 4.

Django's settings file has an important attribute known as INSTALLED_APPS. This is a Python sequence of module names that will be used for the project. In some ways this is Django's secret weapon. Ideally we can deploy dozens of entirely different sites by doing nothing more than creating a new settings file with an appropriate set of modules in INSTALLED_APPS and pointers to different databases and template locations.

Solving small problems with focused Django apps is the best way to achieve these goals. It is important to remember that we will be writing apps, or better yet, normal Python modules. These will be pieces of something larger, not full-blown applications themselves.