code: def id_to_symbol(self, entrez_id):
          
  
          entrez_id = str(entrez_id)
          if entrez_id not in self.ids_to_symbols:
              m = 'Could not look up symbol for Entrez ID ' + entrez_id
              raise Exception(m)
          return self.ids_to_symbols[entrez_id]
docs: Gives the symbol for a given entrez id)