Named cursor problem

Hi,

I am using named cursor in this manner:

cursor = conn.cursor(str(os.getpid))
cursor.execute("select * from documents;")
for row in cursor: print do_something(row) # this function also creates
cursors from "conn"

One million rows have to be processed, but after 10-20 thousands
processed rows I got an error:
Traceback (most recent call last):
   File "/homes/eva/xr/xrylko00/spinn/spinn3r/db/db2xml.py", line 408,
in <module>
     query = query)
   File "/homes/eva/xr/xrylko00/spinn/spinn3r/db/db2xml.py", line 351,
in create_xml
     for item in self.get_item(column, identifiers, query):
   File "/homes/eva/xr/xrylko00/spinn/spinn3r/db/db2xml.py", line 172,
in get_item
     for row in cursor:
ProgrammingError: named cursor isn't valid anymore

Where may be problem?

PostgreSQL 8.4.9 on x86_64-redhat-linux-gnu, compiled by GCC gcc (GCC)
4.1.2 20080704 (Red Hat 4.1.2-51), 64-bit
Python 2.6.5 (r265:79063, Jun 22 2010, 14:35:26)
[GCC 4.3.5] on linux2
 >>> import psycopg2
 >>> psycopg2.__version__
'2.2.2 (dt dec ext pq3)'

Cheers,
Vojtěch R.