Calling a Postgres Function using CGI written in Python

[Fuzzydave]

I did do a google search i have looking through the one python book i
have
and there are plenty of references for how to write a function in
Python but
not as many on how to call a function stored in postgres using python I
have
tried the conn.execute(cmi_grn_cost(productCode)) and also the
conn.callproc(cmi_grn_cost(productCode)) commands where conn is the
connection object and cmi_grn_cost() is the the function getting called
from 
Postgres and productCode is the variable to passed.