Problem with psycopg2, bytea, and memoryview

Frank Millman frank at chagford.com
Wed Jul 31 11:44:01 CEST 2013


Hi all

I don't know if this question is more appropriate for the psycopg2 list, but 
I thought I would ask here first.

I have some binary data (a gzipped xml object) that I want to store in a 
database. For PostgreSQL I use a column with datatype 'bytea', which is 
their recommended way of storing binary strings.

I use psycopg2 to access the database. It returns binary data in the form of 
a python 'memoryview'.

My problem is that, after a roundtrip to the database and back, the object 
no longer compares equal to the original.

>>> memoryview(b'abcdef') == b'abcdef'
True
>>> cur.execute('create table fmtemp (code int, xml bytea)')
>>> cur.execute('insert into fmtemp values (%s, %s)', (1, b'abcdef'))
>>> cur.execute('select * from fmtemp where code =1')
>>> row = cur.fetchone()
>>> row
(1, <memory at 0xb725f77c>)
>>> row[1] == b'abcdef'
False
>>> row[1].tobytes() == b'abcdef'
True
>>>

Using MS SQL Server and pyodbc, it returns a byte string, not a memoryview, 
and it does compare equal with the original.

I can hack my program to use tobytes(), but it would add complication, and 
it would be database-specific. I would prefer a cleaner solution.

Does anyone have any suggestions?

Versions - Python: 3.3.2  PostgreSQL: 9.2.4  psycopg2: 2.5

Frank Millman






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