import sys
import os


import numpy as np
import psycopg2

import commands
import subprocess
#from subprocess import call
import popen2

#from Tkinter import *
#import tkMessageBox

import time
chemin_ogr2ogr = 'C:\OSGeo4W64\bin\'
chemin_saga= 'C:\OSGeo4W64\apps\saga\'

c_serveur_postgres_ip   = 'xxxxxxx'
c_serveur_postgres_port = 'xxxxx'

c_bd_name               = 'xxxxxx'
c_log                   = 'xxxxxx'
c_passw                 = 'xxxxxx'        #

c_schema_obs_obs       = 'rasters'

c_table_entree          = 'tr_rge_alti_5m_2016'

# d?finition des constantes des noms de champs
c_nom_col_geo           = 'rast'

print  'connexion ...'


def connect_db():

    try:
        conn = psycopg2.connect('dbname=' + c_bd_name +' user=' + c_log + ' host=' + c_serveur_postgres_ip + ' password=' + c_passw)
        print conn
        conn.set_session(autocommit=True) #if you want your updates to take effect without being in a transaction and requiring a commit, for a beginner, I would set this to True
        return conn
    except:
        print "I am unable to connect to the database"
        return None

def get_raster(raster_id,conn):

    query= 'SELECT ST_DumpValues('+c_nom_col_geo+', 1 ) from '+c_schema_obs_obs+ '.'+c_table_entree+ ' where rid={}'.format(raster_id)
    conn.cursor().execute(query)
    cur = conn.cursor()
    res = cur.fetchall()

    return res[0][0]

raster_id=0

conn=connect_db()

raster=get_raster(raster_id,conn)

Traceback (most recent call last):
  File "\xxxxxxxprocess_1.py", line 76, in <module>
    raster=get_raster(raster_id,conn)
  File "\xxxxxxxprocess_1.py", line 68, in get_raster
    res = cur.fetchall()
ProgrammingError: no results to fetch