MS Access: silent overflow failure

Hi all,

The small demo script below makes ADO calls to a Microsoft
Access (Jet)
database via win32com. But it fails in a potentially
dangerous way;
"SELECT int * int;" can return None with no
warning if the result is
large enough. This can also occur if one or both of the
operands is a
column reference.

The same SQL run as a Query inside the Access GUI works as
expected.
Manually changing one of the ints to a Single by appending
".0" works as
expected.

Any ideas about where I need to look next to find the cause
of, and then
fix, this behavior?
 

Robert Brewer
System Architect
Amor Ministries
fumanchuamor.org



# demo_access_overflow.py

import os
import win32com.client

adOpenForwardOnly = 0
adLockReadOnly = 1

dbname = "test.mdb"

def create_db():
    cat = win32com.client.Dispatch(r'ADOX.Catalog')
    cat.Create("PROVIDER=MICROSOFT.JET.OLEDB.4.0;DATA
SOURCE=%s;" %
dbname)
    cat.ActiveConnection.Close()

def fetch(query, conn):
    try:
        # Call conn.Open(query) directly, skipping win32com
overhead.
        res = win32com.client.Dispatch(r'ADODB.Recordset')
        res.Open(query, conn, adOpenForwardOnly,
adLockReadOnly)
        data = res.GetRows()
    finally:
        res.Close()
    
    return data


if __name__ == '__main__':
    try:
        create_db()
        conn =
win32com.client.Dispatch(r'ADODB.Connection')
        try:
           
conn.Open("PROVIDER=MICROSOFT.JET.OLEDB.4.0;DATA
SOURCE=%s;"
% dbname)
            
            # This works fine.
            assert fetch("SELECT 86400;", conn) ==
((86400,),)
            
            # This fails because data is ((None,),)
            data = fetch("SELECT 62647 * 86400;",
conn)
            assert (data == ((62647 * 86400,),)), data
            
        finally:
            conn.Close()
    finally:
        if os.path.exists(dbname):
            os.remove(dbname)
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> The small demo script below makes ADO calls to a
Microsoft 
> Access (Jet)
> database via win32com. But it fails in a potentially
dangerous way;
> "SELECT int * int;" can return None with no
warning if the result is
> large enough. This can also occur if one or both of the
operands is a
> column reference.
> 
> The same SQL run as a Query inside the Access GUI works
as expected.
> Manually changing one of the ints to a Single by
appending 
> ".0" works as
> expected.
> 
> Any ideas about where I need to look next to find the
cause 
> of, and then
> fix, this behavior?

I'd suggest using VB to see if the behaviour is different
than with Python.
If it is and we can narrow a test case to a very small
snippet, then we can
probably sort out why.  If VB works the same, it is likely
to be a "feature"
of ADO as implemented, and not much we can do.

Cheers,

Mark

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