pg module defines only a few methods that allow to connect to a database and to define "default variables" that override the environment variables used by PostgreSQL.
   These "default variables" were designed to allow you
   to handle general connection parameters without heavy code in your
   programs. You can prompt the user for a value, put it in the
   default variable, and forget it, without having to modify your
   environment. The support for default variables can be disabled by
   setting the -DNO_DEF_VAR option in the Python
   Setup file. Methods relative to this are specified by the tag [DV].
  
All variables are set to None at module initialization, specifying that standard environment variables should be used.