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pyodbc data corruption problem

P: n/a
I have found a data corruption problem with pyodbc.

OS = Windows XP SP2
DB = Microsoft Access XP

PROBLEM:

When selecting columns from a table that are of type Memo the value
returned is padded with a bunch of null characters at the end.

The problems does not seem to occur until the length of the Memo column
exceeds 2048 bytes.

I have attached several scripts to help demonstrate the problem.

To recreate the problem:

1. Create a blank Access database named test.
2. Create a ODBC DSN named test for that database
3. Run the createtable.py script to create the table
and load it with the dummy data
4. Run the broke.py script to show the problem.

The issue is when the data value is 2048 bytes.

The size in the createtable.py is 2046 bytes plus 3 bytes at the end that
contain "JOE" for a total of 2049 bytes.

If you change it from 2046 to 2045 (or less) then the problem does not
occur.

#
# createtable.py script
#

import pyodbc

dbs = pyodbc.connect('dsn=test')

c = dbs.cursor()

try:
sql = 'drop table test_memo'
c.execute(sql)
dbs.commit()
except:
# ignore drop table failure
pass

sql = 'create table test_memo (c1 int not null, c2 memo not null)'

c.execute(sql)

dbs.commit()

sql = 'insert into test_memo values(1, ?)'

c2_value = '1' * 2046

c2_value = '%sJOE' % (c2_value)

c.execute(sql, (c2_value,))

dbs.commit()

c.close()
dbs.close()

#
# broke.py script
#
import pyodbc

dbs = pyodbc.connect('dsn=test')

c = dbs.cursor()

sql = 'select c2, len(c2) as c2_db_len from test_memo where c1 = 1'

c.execute(sql)

row = c.fetchone()

(
c2,
c2_db_len
) = row

print repr(c2)

print 'c2 length :', len(c2)
print 'c2_db_length :', c2_db_len

print 'before nul length:', len(c2[0:c2.find('\x00')])

c.close()
dbs.close()

May 18 '07 #1
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3 Replies


P: n/a
On May 18, 6:46 pm, "Joe Salmeri" <JoeSalm...@hotmail.comwrote:
I have found a data corruption problem with pyodbc.

OS = Windows XP SP2
DB = Microsoft Access XP

PROBLEM:

When selecting columns from a table that are of type Memo the value
returned is padded with a bunch of null characters at the end.

The problems does not seem to occur until the length of the Memo column
exceeds 2048 bytes.
Interesting. MS-Access has had a bug about Memo fields
ever since Version 2.0 (the last time it worked). I was trying
to use a Memo field on a form and append short status messages.
This mysteriously stopped working in the version following 2.0
(Access 95?) and has never worked since. Seems there is some
kind of 2048 buffer involved that was created by some punk
MicroSoft gave the lowly job of software revision to while
those more educated did more important things.

My guess is you won't find a Python solution short of
accepting what Access gives you and dealing with it.
>
I have attached several scripts to help demonstrate the problem.

To recreate the problem:

1. Create a blank Access database named test.
2. Create a ODBC DSN named test for that database
3. Run the createtable.py script to create the table
and load it with the dummy data
4. Run the broke.py script to show the problem.

The issue is when the data value is 2048 bytes.

The size in the createtable.py is 2046 bytes plus 3 bytes at the end that
contain "JOE" for a total of 2049 bytes.

If you change it from 2046 to 2045 (or less) then the problem does not
occur.

#
# createtable.py script
#

import pyodbc

dbs = pyodbc.connect('dsn=test')

c = dbs.cursor()

try:
sql = 'drop table test_memo'
c.execute(sql)
dbs.commit()
except:
# ignore drop table failure
pass

sql = 'create table test_memo (c1 int not null, c2 memo not null)'

c.execute(sql)

dbs.commit()

sql = 'insert into test_memo values(1, ?)'

c2_value = '1' * 2046

c2_value = '%sJOE' % (c2_value)

c.execute(sql, (c2_value,))

dbs.commit()

c.close()
dbs.close()

#
# broke.py script
#

import pyodbc

dbs = pyodbc.connect('dsn=test')

c = dbs.cursor()

sql = 'select c2, len(c2) as c2_db_len from test_memo where c1 = 1'

c.execute(sql)

row = c.fetchone()

(
c2,
c2_db_len
) = row

print repr(c2)

print 'c2 length :', len(c2)
print 'c2_db_length :', c2_db_len

print 'before nul length:', len(c2[0:c2.find('\x00')])

c.close()
dbs.close()

May 19 '07 #2

P: n/a
Thank you for your response but this is not an Access problem.

The exact same code using mx.ODBC or using the old odbc.py that comes with
the win32 files works fine.

It only fails with pyodbc.

<me********@aol.comwrote in message
news:11*********************@n59g2000hsh.googlegro ups.com...
On May 18, 6:46 pm, "Joe Salmeri" <JoeSalm...@hotmail.comwrote:
>I have found a data corruption problem with pyodbc.

OS = Windows XP SP2
DB = Microsoft Access XP

PROBLEM:

When selecting columns from a table that are of type Memo the value
returned is padded with a bunch of null characters at the end.

The problems does not seem to occur until the length of the Memo column
exceeds 2048 bytes.

Interesting. MS-Access has had a bug about Memo fields
ever since Version 2.0 (the last time it worked). I was trying
to use a Memo field on a form and append short status messages.
This mysteriously stopped working in the version following 2.0
(Access 95?) and has never worked since. Seems there is some
kind of 2048 buffer involved that was created by some punk
MicroSoft gave the lowly job of software revision to while
those more educated did more important things.

My guess is you won't find a Python solution short of
accepting what Access gives you and dealing with it.
>>
I have attached several scripts to help demonstrate the problem.

To recreate the problem:

1. Create a blank Access database named test.
2. Create a ODBC DSN named test for that database
3. Run the createtable.py script to create the table
and load it with the dummy data
4. Run the broke.py script to show the problem.

The issue is when the data value is 2048 bytes.

The size in the createtable.py is 2046 bytes plus 3 bytes at the end that
contain "JOE" for a total of 2049 bytes.

If you change it from 2046 to 2045 (or less) then the problem does not
occur.

#
# createtable.py script
#

import pyodbc

dbs = pyodbc.connect('dsn=test')

c = dbs.cursor()

try:
sql = 'drop table test_memo'
c.execute(sql)
dbs.commit()
except:
# ignore drop table failure
pass

sql = 'create table test_memo (c1 int not null, c2 memo not null)'

c.execute(sql)

dbs.commit()

sql = 'insert into test_memo values(1, ?)'

c2_value = '1' * 2046

c2_value = '%sJOE' % (c2_value)

c.execute(sql, (c2_value,))

dbs.commit()

c.close()
dbs.close()

#
# broke.py script
#

import pyodbc

dbs = pyodbc.connect('dsn=test')

c = dbs.cursor()

sql = 'select c2, len(c2) as c2_db_len from test_memo where c1 = 1'

c.execute(sql)

row = c.fetchone()

(
c2,
c2_db_len
) = row

print repr(c2)

print 'c2 length :', len(c2)
print 'c2_db_length :', c2_db_len

print 'before nul length:', len(c2[0:c2.find('\x00')])

c.close()
dbs.close()


May 19 '07 #3

P: n/a
I have done some additiona investigate into this problem and found the
following:

As described below the problem does not begin to appear until the return
value size is 2048.

Once the return value is greater than 2048 bytes the value returned by
pyodbc is 2 times the actual size of the return value data.

The return value data is padded by exactly the same number of null
characters as there are in the actual data size.

In other words if the actual database value is 4868 bytes, then pyodbc will
return a value that is 9736 bytes long. The first 4868 bytes will be the
real data, followed by 4868 bytes of nulls.

I did a second test where the actual data size was 11,109 bytes. In that
case pyodbc returned a value that was 22,218 bytes long. The first 11,109
bytes are the real data, followed by 11,109 null bytes.

This seems to confirm the bug.

"Joe Salmeri" <Jo********@hotmail.comwrote in message
news:EP******************************@comcast.com. ..
>I have found a data corruption problem with pyodbc.

OS = Windows XP SP2
DB = Microsoft Access XP

PROBLEM:

When selecting columns from a table that are of type Memo the value
returned is padded with a bunch of null characters at the end.

The problems does not seem to occur until the length of the Memo column
exceeds 2048 bytes.

I have attached several scripts to help demonstrate the problem.

To recreate the problem:

1. Create a blank Access database named test.
2. Create a ODBC DSN named test for that database
3. Run the createtable.py script to create the table
and load it with the dummy data
4. Run the broke.py script to show the problem.

The issue is when the data value is 2048 bytes.

The size in the createtable.py is 2046 bytes plus 3 bytes at the end that
contain "JOE" for a total of 2049 bytes.

If you change it from 2046 to 2045 (or less) then the problem does not
occur.

#
# createtable.py script
#

import pyodbc

dbs = pyodbc.connect('dsn=test')

c = dbs.cursor()

try:
sql = 'drop table test_memo'
c.execute(sql)
dbs.commit()
except:
# ignore drop table failure
pass

sql = 'create table test_memo (c1 int not null, c2 memo not null)'

c.execute(sql)

dbs.commit()

sql = 'insert into test_memo values(1, ?)'

c2_value = '1' * 2046

c2_value = '%sJOE' % (c2_value)

c.execute(sql, (c2_value,))

dbs.commit()

c.close()
dbs.close()

#
# broke.py script
#
import pyodbc

dbs = pyodbc.connect('dsn=test')

c = dbs.cursor()

sql = 'select c2, len(c2) as c2_db_len from test_memo where c1 = 1'

c.execute(sql)

row = c.fetchone()

(
c2,
c2_db_len
) = row

print repr(c2)

print 'c2 length :', len(c2)
print 'c2_db_length :', c2_db_len

print 'before nul length:', len(c2[0:c2.find('\x00')])

c.close()
dbs.close()

May 24 '07 #4

This discussion thread is closed

Replies have been disabled for this discussion.