The cgi.FieldStorage object has a headers property,
defined as:

"headers         : header dictionary-like object; default:
    taken from environ as per CGI spec"

But the CGI Spec gives a list of well-known environment
variables, plus a generic method to read environment
variables (those starting with HTTP_), that are *not*
read and parsred in this dictionary.
http://hoohoo.ncsa.uiuc.edu/cgi/env.html