了解继承

Question

我试图更好地理解继承。在下面的代码中，当我尝试打印friend.salary时，它会抛出一个AttributeError。WorkingStudent不是继承Student类的所有方法吗？

class Student:
def __init__(self,name,school):
self.name = name
self.school = school
self.marks = []
def average(self):
return sum(self.marks)/len(self.marks)
def friend(self,friend_name):
return Student(friend_name, self.school)
anna = Student("Anna","MIT")
friend = anna.friend("Motilal")
#print (friend.name)
#print (friend.school)
class WorkingStudent(Student):
def __init__(self,name,school,salary):
super().__init__(self,name,school)
self.salary = salary
anna = WorkingStudent("Anna","SXS",25000)
anna.friend("Greg")
anna.marks.append(50)
print friend.salary 

Answer 1

你应该修改你的源代码，如下所示

class Student:
def __init__(self,name,school):
self.name = name
self.school = school
self.marks = []
def average(self):
return sum(self.marks)/len(self.marks)
def friend(self,friend_name):
return Student(friend_name, self.school)

anna = Student("Anna","MIT")
friend = anna.friend("Motilal")
#print (friend.name)
#print (friend.school)

class WorkingStudent(Student):
def __init__(self,name,school,salary):
super(WorkingStudent,self).__init__(name,school)
self.salary = salary
# anna = WorkingStudent("Anna","SXS",25000)
def friend(self,friend_name):
return WorkingStudent(friend_name, self.school, self.salary)
# You should put your code here, because as your original code
# anna is an instance of Student not WorkingStudent class
# so it and its friend don't have "salary".
anna = WorkingStudent("Anna","SXS",25000) # After this line, anna is a different variable to "anna" variable that was defined before (anna = Student("Anna","MIT"))
friend = anna.friend("Greg") # friend now is an instance of WorkingStudent class, so it have salary
anna.marks.append(50)
print(friend.salary)

已编辑。所以代码现在可以工作了