PyMOTW: sys, Part 7: Modules and Imports
Most Python programs end up as a combination of several modules with a main application importing them. Whether you are using the features of the standard library, or organizing your own code in separate files to make it easier to maintain, understanding and managing the dependencies for your program is an important aspect of development.
sys includes information about the modules available to your application, either as built-ins or after being imported. It also defines hooks for overriding the standard import behavior for special cases.