Python Language Classes Basic inheritance
Example
Inheritance in Python is based on similar ideas used in other object oriented languages like Java, C++ etc. A new class can be derived from an existing class as follows.
class BaseClass(object):
pass
class DerivedClass(BaseClass):
pass
The BaseClass is the already existing (parent) class, and the DerivedClass is the new (child) class that inherits (or subclasses) attributes from BaseClass. Note: As of Python 2.2, all classes implicitly inherit from the object class, which is the base class for all built-in types.
We define a parent Rectangle class in the example below, which implicitly inherits from object:
class Rectangle():
def __init__(self, w, h):
self.w = w
self.h = h
def area(self):
return self.w * self.h
def perimeter(self):
return 2 * (self.w + self.h)
The Rectangle class can be used as a base class for defining a Square class, as a square is a special case of rectangle.
class Square(Rectangle):
def __init__(self, s):
# call parent constructor, w and h are both s
super(Square, self).__init__(s, s)
self.s = s
The Square class will automatically inherit all attributes of the Rectangle class as well as the object class. super() is used to call the __init__() method of Rectangle class, essentially calling any overridden method of the base class. Note: in Python 3, super() does not require arguments.
Derived class objects can access and modify the attributes of its base classes:
r.area()
# Output: 12
r.perimeter()
# Output: 14
s.area()
# Output: 4
s.perimeter()
# Output: 8
Built-in functions that work with inheritance
issubclass(DerivedClass, BaseClass): returns True if DerivedClass is a subclass of the BaseClass
isinstance(s, Class): returns True if s is an instance of Class or any of the derived classes of Class
# subclass check
issubclass(Square, Rectangle)
# Output: True
# instantiate
r = Rectangle(3, 4)
s = Square(2)
isinstance(r, Rectangle)
# Output: True
isinstance(r, Square)
# Output: False
# A rectangle is not a square
isinstance(s, Rectangle)
# Output: True
# A square is a rectangle
isinstance(s, Square)
# Output: True
