***can't delete function call(my_dictionary.py, line 21)

Имеется такое:
# -*- coding: cp1251 -*-
class Phone_dict:
     
      def __init__(self):         
          self.f = open('****','rw')
          self.dict = {}
          for line in self.f:
                self.dict[line.split(':')[0].strip()] = line.split(':')[1].strip()
          self.f.close()

      def fill_dict_phone(self,dname,phone):
            self.dict[str(dname)] = str(phone)
      def close(self):
            self.f.close()
      def show(self):
            for name in self.dict.keys():
                  print str(name) +" : "+ str(self.dict[name])
      def delete(self):
            self.dict.clear()
      def del_phone(self,name):
            del self.dict(name)
      def save_to(self):
            self.f = open('***','w')
            for i in self.dict.keys():
                  self.f.write(str(i)+':'+str(self.dict[i])+"\n")
            self.f.close()

if __name__=='__main__':
      d = Phone_dict()
При выполнении пишет "***can't delete function call(my_dictionary.py, line 21)"
21 строка - это та, которую я выделил.
Подскажите, в чем проблема...