mixins in python

Topics

• oops

Mixins are Python’s way to interfaces - classes that define behavior. Although a mixin is defined just like any other class, there are some “best practices” to follow:

• Mixins should be used to add a “behavior” to another class. With this in mind, there’s no need for a mixin to have state variables
• Since mixins are classes, they can also inherit from other classes, but it’s advisable to keep them self-contained and not inherit from other mixins/classes

A mixin is a class with a single purpose to add functionality to other classes. Multiple inheritance is used to achieve mixins in Python.

We can see how they help with an example:

@dataclass
class Coord:
    x: float
    y: float
 
@dataclass
class Center(Coord):
    "Center of shape"
    pass
 
@dataclass
class TopLeft(Coord):
    "Top left 'corner' of a shape"
    pass
 
@dataclass
class Shape:
    coord: Coord
 
# ========================== #
# Above is part of a library (say)
# and can't be modified
# ========================== #
 
class Rectangle(Shape):
    def __init__(self, x, y, w, h):
        super().__init__(TopLeft(x, y))
        self.w = w
        self.h = h
 
class Circle(Shape):
    def __init__(self, x, y, r):
        super().__init__(Center(x, y))
        self.r = r
 
 
rect = Rectangle(0, 0, 100, 50)
circle = Circle(10, 10, 50)
 
 
print(rect.coord) # TopLeft(x=0, y=0)
print(circle.coord) # Center(x=10, y=10)

In above toy example, we have different shapes like Rectangle and Circle. Say, I want to have the ability to serialize them. Since we can’t modify Shape class as it’s “locked” in a library, we would normally create a serialize method within the subclasses and call it. Within mixins, things get simpler. Since serializing is a behavior, we can have a SerializerMixin:

from dataclasses import asdict
 
class SerializerMixin:
    def serialize(self):
        print(asdict(self))
 
class Rectangle(Shape, SerializerMixin):
    ...
 
class Circle(Shape, SerializerMixin):
    ...
 
circle.serialize() # {'coord': {'x': 10, 'y': 10}}

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