Python client + select = locked resources???

Hi!

I wanna ask something. I came from IB/FB world.
In this world I was everytime in transaction, because of reads are also
working under transactions.
In the FB world the transactions without any writes/updates are not
locking the database, so another clients can makes a transactions on any
records.
And also can add new fields to the tables.

Now I used Pylons webserver (Python) with PyGRESQL, and DBUtils for
cached database connections/cursors.

Today I saw a locking situation in many times.

0.) I started Pylons web server, and in the browser I request for a
simple view (without modify anything).
1.) I opened PGAdmin.
2.) I move the focus to the table "X".
3.) I opened an SQL editor and try to make two column adds:
alter table X add test_a date;
alter table X add test_b date;
4.) After the the PGAdmin's Query Execution (F5) nothing happened. I see
this menu is disabled, and PGAdmin is locked for new operations.
5.) When I simply close "Pylons web server", the PGAdmin quickly
finished with this table restructure without problems...

The problem can repeatable in any times.

This is very hateable thing, because in this view I don't modify
anything, I use only "selects", nothing other things.

And I wanna solve this problem, because if I must do some modifications
in the online database (for example: add a new field), I don't want to
shut down the webserver with all online clients...

I simplified this "bug" to see this without web server, dbutils, and
other layers.

I wrote this python code:

import os, sys, pgdb

fmtstring = '%s:%s:%s:%s'
fmtstring = fmtstring % ('127.0.0.1',
'anydb',
'anyuser', 'what?')
db = pgdb.connect (fmtstring)
print "ok"
cur = db.cursor()
cur.execute('select * from testtable')
rek = cur.fetchone()
cur.close()
while 1:
pass
db.close()

After start this I tried to add a new field to the testtable from
PGAdmin's Query GUI:
alter table testtable add test_001 date;

With the cur.execute("select * from testtable") I got lock "error", the
PGAdmin query is running and running and running... :-(

Without this cur.execute the alter table finished without locks.

When I put a "db.rollback()" before while the lock vanished...

So pg is hold all records I fetched? They are not useable freely in a
simple, readonly select?

Please help me SOS if possible, I must find a way to get out from these
locks!

And please help me: how to check that I'm in transaction or not?

Thanks for your help:
dd