1   class MyClass:
 2     def __init__(self):
 3       self._x = 1
 4         
 5     def increase(self):
 6       self._x += 1
 7           
 8   class MyOtherClass(class MyClass):     # proper declaration of parent class
 9     def __init__(self, z):
10       MyClass.__init__(self)             # must send self reference to parent constructor
11       self._x = z
12         
13     def decrease(self):
14       self._x -= 1                       # must use self reference to access inherited attribute
15           
16   a = MyClass()
17   b = MyOtherClass(42)                   # constructor expects an actual parameter for z value
18   b.increase()
19   a.decrease()                           # MyClass instance does not support decrease method

class Student(Person):
  def __init__(self, name, age, id):
    Person.__init__(self, name, age)
    self.id = id
      
  def printInfo(self):
    Person.printInfo(self)
    print 'ID:', self.id

This code would be flawed. By invoking extend directly upon the list instance rather than with our SortedSet version of that method, duplicate entries are not removed and the initial values are not automatically sorted.

In this case, the code remains legitimate. The syntax len(self) is equivalent to len(self._items) based upon our implementation of SortedSet.__len__ Similarly, self[walk] is equivalent in result to self._items[walk] due to our implementation of the SortedSet.__getitem__ method.