CREATE POLICY test_data_pol ON test_data FOR ALL USING ('conc' = get_pol())

CREATE OR REPLACE FUNCTION public.set_pol(foo_val integer)
  RETURNS void AS
$BODY$
BEGIN
  EXECUTE format('SET SESSION my.foo TO %I', foo_val) ;
END;
$BODY$

CREATE OR REPLACE FUNCTION public.get_pol()
  RETURNS character varying AS
$BODY$
DECLARE
  foo_val character varying;
BEGIN
  SHOW my.foo INTO foo_val;
  RETURN foo_val;
END;
$BODY$

import psycopg2
conn = psycopg2.connect(host=DATABASE['host'],
                        database=DATABASE['dbname'],
                        user=DATABASE['user'],
                        password=DATABASE['password'],
                        port=DATABASE['port'])
cursor = conn.cursor()
cursor.execute("SELECT set_pol(%s)" % conc_value)
cursor.execute("""SELECT * from test_data""")
data = cursor.fetchall()

uri = QgsDataSourceURI()
uri.setConnection(host,
                   port,
                   dbname,
                   user,
                   pasword)
uri.setParam('my.foo', 'conc_value')
uri.setDataSource('public', 'test_data', None, None, 'id')
lyr = QgsVectorLayer(uri.uri(), 'test_data', 'postgres')

ERROR:  unrecognized configuration parameter "my.foo"
CONTEXT:  SQL statement "SHOW my.foo"
PL/pgSQL function get_pol() line 5 at SQL statement
.
SQL : SELECT * FROM "public"."test_data" LIMIT 1