Machine:  RHEL 5.3 x86_64 / Python 2.7.3 / pyodbc-3.0.3
Database: IBM Netezza TwinFin Appliance
ODBC:     NZ odbc client for Linux

Instance
    create table tst(a double, b numeric(9,4), c varchar(10), d char(4));
    insert into tst select 1.01, 1.01, 'aa', 'aa';

    select * from tst;
      A   |   B    | C  |  D
    ------+--------+----+------
     1.01 | 1.0100 | aa | aa

TEST
    import pyodbc
    conn = pyodbc.connect("DSN=vntest")
    cursor = conn.cursor()

Test 1
    cursor.execute("select a,c,d from tst")
    for row in cursor:
        print row

OUTPUT: (1.01, 'aa', 'aa  ')

Test 2
    cursor.execute("select b from tst")
    for row in cursor:
        print row

OUTPUT:
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/local/Python-2.7.3/lib/python2.7/decimal.py", line 548, in
__new__
        "Invalid literal for Decimal: %r" % value)
      File "/usr/local/Python-2.7.3/lib/python2.7/decimal.py", line 3866,
in _raise_error
        raise error(explanation)
    decimal.InvalidOperation: Invalid literal for Decimal: u''

I would appreciate any help with this.  Thanks.