Re: psycopg3 and cur.description behavior

On Sun, 3 Oct 2021 at 15:33, Paolo De Stefani <paolo(at)paolodestefani(dot)it>
wrote:
>
> Hello psycopg users
> This is my first post on this mailing list

Hello Paolo, welcome here.

> In my python code i use the return query to create a dictionary in this
> way:
>
> session.update(dict(zip([i[0] for i in cur.description],
> cur.fetchall()[0])))
>
> This part no longer works in psycopg 3. Looks like cur.description in
> psycopg 3 is different if i execute a query or call a function.

Are you sure you are using the same statements in psycopg 2 and 3? If you
call `select pa_connect` or `select * from pa_connect` you get different
results: a table with a single column of records in the first case,
expanded records in the second. You can verify that in psql too. Using a
simplified set returning function:

piro=# create or replace function testfunc() returns table(pid int, type
text) language plpgsql as $$
begin
return query select a.pid, a.backend_type from pg_stat_activity a;
end$$;

piro=# select * from testfunc() limit 3;
┌────────┬──────────────────────────────┐
│ pid │ type │
├────────┼──────────────────────────────┤
│ 625446 │ autovacuum launcher │
│ 625448 │ logical replication launcher │
│ 806502 │ client backend │
└────────┴──────────────────────────────┘
(3 rows)

piro=# select testfunc() limit 3;
┌─────────────────────────────────────────┐
│ testfunc │
├─────────────────────────────────────────┤
│ (625446,"autovacuum launcher") │
│ (625448,"logical replication launcher") │
│ (806502,"client backend") │
└─────────────────────────────────────────┘
(3 rows)

Psycopg would see pretty much the same: in psycopg2 you obtain two columns
if you use "select * from", only one "record" column if you don't:

In [1]: import psycopg2
In [2]: cnn = psycopg2.connect("")
In [3]: cur = cnn.cursor()

In [4]: cur.execute("select * from testfunc()")
In [5]: cur.description
Out[5]: (Column(name='pid', type_code=23), Column(name='type',
type_code=25))
In [6]: cur.fetchone()
Out[6]: (625446, 'autovacuum launcher')

In [7]: cur.execute("select testfunc()")
In [8]: cur.description
Out[8]: (Column(name='testfunc', type_code=2249),)
In [9]: cur.fetchone()
Out[9]: ('(625446,"autovacuum launcher")',)

Psycopg 3 returns something similar:

In [1]: import psycopg
In [2]: cnn = psycopg.connect("")

In [3]: cur = cnn.execute("select * from testfunc()")
In [4]: cur.description
Out[4]: [<Column 'pid', type: int4 (oid: 23)>, <Column 'type', type: text
(oid: 25)>]
In [5]: cur.fetchone()
Out[5]: (625446, 'autovacuum launcher')

In [6]: cur = cnn.execute("select testfunc()")
In [7]: cur.description
Out[7]: [<Column 'testfunc', type: record (oid: 2249)>]
In [8]: cur.fetchone()
Out[8]: (('625446', 'autovacuum launcher'),)

There is a difference in how the record is handled: psycopg 2 doesn't parse
it, psycopg 3 unpacks it in a tuple (although it doesn't have enough info
to understand the types contained in the record, so they are left as
strings). However the number and oids of the columns in the result is the
same. The pattern you use to convert the record into a dict should work the
same way in psycopg 3 too:

In [9]: cur = cnn.execute("select * from testfunc()")

In [10]: dict(zip([i[0] for i in cur.description],cur.fetchall()[0]))
Out[10]: {'pid': 625446, 'type': 'autovacuum launcher'}

> BUT if i execute (call) the already mentioned postgresql function:
>
> cur.execute('SELECT system.pa_connect(%s, %s, %s, %s, %s, %s);',
> ...

I think you want to use `SELECT * FROM system.pa_connect(...)` here, and I
think it is what you were using before.

Does it make sense?

Once you are comfortable with how the types of query work, you might want
to take a look at 'dict_row()' (
https://www.psycopg.org/psycopg3/docs/advanced/rows.html) to convert
records to dicts in a more succinct way:

In [11]: from psycopg.rows import dict_row

In [12]: cur = cnn.cursor(row_factory=dict_row)

In [13]: cur.execute("select * from testfunc()").fetchone()
Out[13]: {'pid': 625446, 'type': 'autovacuum launcher'}

Cheers

-- Daniele