#比如三个class，士兵，老师，学生，有共性：姓名，性别，年龄；有个性：士兵有军衔，老师有课程，学生有成绩
#当然可以用三个不同的class来包含共性和个性，但是更方便的是把共性放到一个parent class里面，
#然后各个个性放到不同的children class里面
class Person(object):
 def __init__(self, name, age):
  self.name = name
  self.age = age
 def __str__(self):
  print "Person -"
  return "My name is %s. I am %s years old" %(self.name, self.age) 

class Military(Person):
 def __init__(self, name, age, rank):
  Person.__init__(self, name, age)    #call Person class method
  self.rank = rank
 def __str__(self):    #it will overwrites the parent __str__ method
  print "Military -"
  return Person.__str__(self) + "I am a %s" %(self.rank)

class Teacher(Person):
 def __init__(self, name, age, subject):
  Person.__init__(self, name, age)    #call Person class method
  self.subject = subject
 def __str__(self):    #it will overwrites the parent __str__ method
  print "Teacher -"
  return Person.__str__(self) + "I teach %s" %(self.subject)

class Student(Person):
 def __init__(self, name, age, loans):
  Person.__init__(self, name, age)    #call Person class method
  self.loans = loans

person = Person('Mike', 20)
print person
m1 = Military('Dan', 20, 'Soldier')
print m1
t1 = Teacher('Mike', 35, 'Math')
print t1
s1 = Student('Ted', 20, 10000)
print s1