A string giving the site-specific directory prefix where the platform-dependent Python files are installed; by default, this is also '/usr/local'. This can be set at build time with the --exec-prefix argument to the configure script. Specifically, all configuration files (e.g. the pyconfig.h header file) are installed in the directory exec_prefix/lib/pythonX.Y/config, and shared library modules are installed in exec_prefix/lib/pythonX.Y/lib-dynload, where X.Y is the version number of Python, for example 3.2.
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).