from osgeo import ogr

filepath = 'C:/Users/username/Desktop/path/clip.shp' # set the filepath
layer_name = filepath[:-4].split('/')[-1] # get the layer name
driver = ogr.GetDriverByName ("ESRI Shapefile")
ogr_ds = driver.Open(filepath)
TEST=2
sql = "SELECT myfield FROM %s WHERE OBJECTID=%s" %(layer_name,TEST)
layer = ogr_ds.ExecuteSQL(sql)
feat = layer.GetNextFeature()
val = feat.GetField(0)

print (val.decode('utf-8'))

import psycopg2

try:
    conn = psycopg2.connect("dbname='template1' user='dbuser' host='localhost' password='dbpass'")
except:
    print "I am unable to connect to the database"

c = conn.cursor()
c.execute('SELECT * FROM mytesttable')
c.fetchall()

c.execute('COMMIT;')

def get_dbparams(self):
    # Get db connections params #
    DB = 'somedb'
    PORT = 5432
    HOST = '11.22.333.44'

    user = self.get_userrole()

    if user == GrukosUser.READER:
        db_params = {
            'dbname': DB,
            'user': 'grukosreader',
            'host': HOST,
            'password': base64.b64decode('SDFADSASDFADF'),
            'port': PORT
        }
        return db_params
    elif user == GrukosUser.WRITER:
        db_params = {
            'dbname': DB,
            'user': 'grukoswriter',
            'host': HOST,
            'password': base64.b64decode('SDFGSGSDFGSDFGSDF'),
            'port': PORT
        }
        return db_params
    else:
        return None

db = MyDb()
    params = db.get_dbparams()

uri = QgsDataSourceURI()
uri.setConnection(params['host'], str(params['port']), params['dbname'], params['user'], params['password'])

uri.setDataSource("myschema", "mytablename", None, "mycolumn={}".format(myvalue))

uri.setDataSource("myschema", "mytablename", "mygeom", "mycolumn={}".format(myvalue))

uri.setDataSource("myschema", "mytablename", None)

uri.setDataSource("myschema", "mytablename", "geom")

uri.setDataSource("myschema", "mytablename", "geom", aKeyColumn='id')
uri.setKeyColumn('id')

vlayer = QgsVectorLayer(uri.uri(), "mylayername", "postgres")

if not vlayer.isValid():
   ...

QgsMapLayerRegistry.instance().addMapLayer(vlayer, True)

def get_projectid(self, gkoid):
    """ Use a PostgreSQL function to translate gkoid to projectid """
    sql = 'SELECT myschema.mypgfunction({});'.format(gkoid)
    cur = self.execute_sql(sql)
    rows_tuple = cur.fetchone()
    projectid = rows_tuple[0]

    return projekcid, sql

def execute_sql(self, sql, dict_cursor=True, print_sql=False):

    """ Execute a SQL query
    :param sql: SQL to be executed
    :param dict_cursor: Flag indicating if cursor is a dict or not. Use false for scalar queries
    :param print_sql: Flag indicating if sql is to be printet
    :return: returns a cursor
    """

    if print_sql: print sql
    conn = psycopg2.connect(**self.get_dbparams())

    if dict_cursor:
        cur = conn.cursor(cursor_factory = psycopg2.extras.DictCursor)
    else:
        cur = conn.cursor()

    try:
        cur.execute(sql)
        return cur
    except psycopg2.DatabaseError, e:
        print 'Some error {}'.format(e)
        sys.exit(1)
    finally:
        pass
        #TODO
        #if conn:
        #    conn.close()

def enable_qgis_log(filename = 'D:gqis.log', haltApp = False, haltMsg = 'stop'):
    """ Very useful when QGIS crashes on PGSQL error
    :param filename: Filename and path for log file
    :param haltApp: Halts the application with a modal dialog
    :param haltMsg: Message to user when showing model stopping dialog
    :rtype: None
    """
    def write_log_message(message, tag, level):
        with open(filename, 'a') as logfile:
            logfile.write('{tag}({level}): {message}'.format(tag=tag, level=level, message=message))

    QgsMessageLog.instance().messageReceived.connect(write_log_message)

    if haltApp:
        QtGui.QMessageBox.information(None, GrukosAux.GRUKOS, "{}".format(haltMsg.encode('cp1252')), QtGui.QMessageBox.Ok)