An underlying philosophy in the development of Sage is that it should provide unified library-level access to the some of the best GPL’d C/C++ libraries. Currently Sage provides some access to MWRANK, NTL, PARI, and Hanke, each of which are included with Sage.
The interfaces are implemented via shared libraries and data is moved between systems purely in memory. In particular, there is no interprocess interpreter parsing (e.g., expect), since everything is linked together and run as a single process. This is much more robust and efficient than using expect.
Each of these interfaces is used by other parts of Sage. For example, mwrank is used by the elliptic curves module to compute ranks of elliptic curves, and PARI is used for computation of class groups. It is thus probably not necessary for a casual user of Sage to be aware of the modules described in this chapter.