title: Not able to connect to a table using python cx_oracle
body: <p>I'm getting the mentioned error while I'm using a WHERE condition in cx_oracle.
  However, there is no error while I fetch all rows.
  Please find my code below. The input - cols is a list of all the columns I want to fetch. Similarly, I want to pass the where condition as variable too, wherein I'm passing on the values in a loop.</p>
  <p>For testing, lets keep it static.</p>
  <p>Thanks</p>
  <pre><code> cols = [
          'ID',
          'CODE',
          'LOGINDEX_CODE',
          'IS_ACTIVE',
          'IS_PORT_GROUP'
          ]
  table_name = 'PORT'
  
  os.chdir(os.path.dirname(__file__))
  main_dir = os.getcwd()
  
  import cx_Oracle
  
  try:
      cx_Oracle.init_oracle_client(lib_dir= main_dir + &quot;\\instantclient_21_3\\&quot;)
  except:
      pass
  
  dsn_tns = cx_Oracle.makedsn(r'some_db', 1521, service_name=r'some_service')
  conn = cx_Oracle.connect(user='abcdef', password=r'ghijkl', dsn=dsn_tns).cursor()
  
  query_cols = ', '.join(cols)
  named_params = {
                  'varx'  :   query_cols,
                  'vary'  :   'LEG',
                  'varz'  :   242713
                  }
  
  
  sql_query = 'SELECT :varx FROM :vary WHERE START_PORT_ID IN :varz'
  conn.prepare(sql_query)
  conn.execute(sql_query, named_params)
  </code></pre>