Python supports inheritance from multiple classes. In this lesson, you’ll see:
- How multiple inheritance works
- How to use
super()to call methods inherited from multiple parents - What complexities derive from multiple inheritance
- How to write a mixin, which is a common use of multiple inheritance
A class can inherit from multiple parents. For example, you could build a class representing a 3D shape by inheriting from two 2D shapes:
class RightPyramid(Triangle, Square):
def __init__(self, base, slant_height):
self.base = base
self.slant_height = slant_height
def what_am_i(self):
return 'RightPyramid'
The Method Resolution Order (MRO) determines where Python looks for a method when there is a hierarchy of classes. Using super() accesses the next class in the MRO:
class A:
def __init__(self):
print('A')
super().__init__()
class B(A):
def __init__(self):
print('B')
super().__init__()
class X:
def __init__(self):
print('X')
super().__init__()
class Forward(B, X):
def __init__(self):
print('Forward')
super().__init__()
class Backward(X, B):
def __init__(self):
print('Backward')
super().__init__()
If you combine the MRO and the **kwargs feature for specifying name-value pairs during construction, you can write code that passes parameters to parent classes even if they have different names:
class Rectangle:
def __init__(self, length, width, **kwargs):
self.length = length
self.width = width
super().__init__(**kwargs)
def area(self):
return self.length * self.width
def perimeter(self):
return 2 * self.length + 2 * self.width
class Square(Rectangle):
def __init__(self, length, **kwargs):
super().__init__(length=length, width=length, **kwargs)
class Triangle:
def __init__(self, base, height, **kwargs):
self.base = base
self.height = height
super().__init__(**kwargs)
def tri_area(self):
return 0.5 * self.base * self.height
class RightPyramid(Square, Triangle):
def __init__(self, base, slant_height, **kwargs):
self.base = base
self.slant_height = slant_height
kwargs["height"] = slant_height
kwargs["length"] = base
super().__init__(base=base, **kwargs)
def area(self):
base_area = super().area()
perimeter = super().perimeter()
return 0.5 * perimeter * self.slant_height + base_area
def area_2(self):
base_area = super().area()
triangle_area = super().tri_area()
return triangle_area * 4 + base_area
Multiple inheritance can get tricky quickly. A simple use case that is common in the field is to write a mixin. A mixin is a class that doesn’t care about its position in the hierarchy, but just provides one or more convenience methods:
class SurfaceAreaMixin:
def surface_area(self):
surface_area = 0
for surface in self.surfaces:
surface_area += surface.area(self)
return surface_area
class Cube(Square, SurfaceAreaMixin):
def __init__(self, length):
super().__init__(length)
self.surfaces = [Square, Square, Square, Square, Square, Square]
class RightPyramid(Square, Triangle, SurfaceAreaMixin):
def __init__(self, base, slant_height):
self.base = base
self.slant_height = slant_height
self.height = slant_height
self.length = base
self.width = base
self.surfaces = [Square, Triangle, Triangle, Triangle, Triangle]
Here’s what you get:
>>> cube = Cube(3)
>>> cube.surface_area()
54
Richard Morris on Jan. 14, 2020
Superbly motivated, organized, and paced. Most appreciated