Обсуждение: Problem with memoryview

Hi all

The following refers to Python 3.3.

I know that if you create a column of type 'bytea', psycopg2 accepts a
variety of inputs, but always returns a 'memoryview'.

I would like to know if it is possible, through some customisation option,
to tell it to return 'bytes'.

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
>>>

TIA

Frank Millman

On Wed, Jul 31, 2013 at 3:08 PM, Frank Millman <frank@chagford.com> wrote:
>
> The following refers to Python 3.3.
>
> I know that if you create a column of type 'bytea', psycopg2 accepts a
> variety of inputs, but always returns a 'memoryview'.
>
> I would like to know if it is possible, through some customisation option,
> to tell it to return 'bytes'.

Yes: you can easily create a typecaster converting bytea data to bytes:

    def bytea2bytes(value, cur):
        m = psycopg2.BINARY(value, cur)
        if m is not None:
            return m.tobytes()

    BYTEA2BYTES = psycopg2.extensions.new_type(
        psycopg2.BINARY.values, 'BYTEA2BYTES', bytea2bytes)
    psycopg2.extensions.register_type(BYTEA2BYTES)

    cur.execute("select 'abcdef'::bytea")
    cur.fetchone()[0]    # returns b'abcdef'

This would change the behaviour of bytea globally; you can limit the
scope of the change to a single connection or cursor passing this
object as parameter to register_type(): check the function docs for
details.

Hope it helps,


-- Daniele

----- Original Message -----
From: "Daniele Varrazzo" <daniele.varrazzo@gmail.com>
To: "Frank Millman" <frank@chagford.com>
Cc: <psycopg@postgresql.org>
Sent: Wednesday, July 31, 2013 4:56 PM
Subject: Re: [psycopg] Problem with memoryview


> On Wed, Jul 31, 2013 at 3:08 PM, Frank Millman <frank@chagford.com> wrote:
>>
>> The following refers to Python 3.3.
>>
>> I know that if you create a column of type 'bytea', psycopg2 accepts a
>> variety of inputs, but always returns a 'memoryview'.
>>
>> I would like to know if it is possible, through some customisation
>> option,
>> to tell it to return 'bytes'.
>
> Yes: you can easily create a typecaster converting bytea data to bytes:
>
[..]

Thanks very much, Daniele. I have tested it, and it works like a charm :-)

Frank