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.

Read more at sys