A string giving the site-specific directory prefix where the platform independent Python files are installed; on Unix, the default is /usr/local. This can be set at build time with the --prefix argument to the configure script. See Installation paths for derived paths.
3.14: When running under a virtual environment, prefix and exec_prefix are now set to the virtual environment prefix by the path initialization, instead of site. This means that prefix and exec_prefix always point to the virtual environment, even when site is disabled (-S).