Function not inserting rows

Hi,

i have the following question:

Given an empty database with only schema api_dev in it, a table and a
function is created as follows:

CREATE TABLE api_dev.item_texts
(
item_id integer,
item_text text
)
WITH (
OIDS=FALSE
);

CREATE OR REPLACE FUNCTION api_dev.add_texts_to_item(
p_item_id integer,
p_item_texts text[])
RETURNS boolean AS
$BODY$

BEGIN

insert into api_dev.item_texts( item_id, item_text )
(
select p_item_id, unnest( p_item_texts )
);
return true;

END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

When i call this function in pgadmin (3, 1.22.2) like this:

select * from api_dev.add_texts_to_item( 444, array['PGADM1', 'PGADM2'] );

i get the true-result and the table will have two rows:

444, PGADM1
444, PGADM2

Now (this is NOT a Python question), when i connect with the same user via
Python psycopg2 to the same database via the following function:

def add_texts_to_item( self, item_id, texts ):
sql = "select * from api_dev.add_texts_to_item( %s, %s );"
self.cur_.execute( sql, (doc_id, isins, ) )
data = self.cur_.fetchone()

if data is None:
return None

return data[0]

I will also get the true result, but nothing is being added. But the
SQL-Statement that gets to the DB is identical. When i force a syntax error
into the statement to see the actual statement it creates, like this:

sql = "select * from api_dev.add_texts_to_item( %s, %s ); x x"
i get the following python-error:
psycopg2.ProgrammingError: FEHLER: Syntaxfehler bei »s«
LINE 1: ...dd_texts_to_item( 1234, ARRAY['PSYCOPG1', 'PSYCOPG2'] ); s s

But the created statement looks syntax-wise identical to the
pgadmin-statement (except for the forced error of course):

select * from api_dev.add_texts_to_item( 444, array['PGADM1', 'PGADM2'] );

When i change the return type of the pgsql-function from true to false, i
will also get the respective result back in Python, so it is actually
calling the psql-function and i can also see it in the logs, but nothing
gets inserted.

I noticed this behavior first in a Linux 64 bit / 9.6.3 machine and then
reproduced this isolated sample on Windows 7 64 bit/ 9.6.2 version.

Any ideas ?

Thanks