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.