Mixin vs inheritance
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What is the difference between a mixin and inheritance?

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edited Jan 7 '12 at 21:18
user142019
asked May 13 '09 at 20:31

Johnd
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A Mix in is typically used with multiple inheritance. So, in that sense, there's "no difference".

The detail is that a Mix in is rarely useful as a standalone object.

For example, say you have a Mix In name "ColorAndDimension", which adds a color property and width and height.

Now, you could add ColorAndDimension to a, say, Shape Class, a Sprite Class, a Car Class, etc. And they will all have the same interface (say get/setColor, get/setHeight/Width, etc.)

So, in the generic case a Mix in IS inheritance. But you can argue it's a matter of the role of the class in the overall domain as to whether a Mix in is a "primary" class or simply a mix in.

Edit -- just to clarify.

Yes, a Mix In can be considered, in todays modern lingo, an Interface with an associated Implementation. It really is just plain, old, everyday multiple inheritance using a plain, old, everyday class. It just happens to be a specific application of MI. Most languages don't give a Mix In any special status, it's just a class that was designed to be "mixed in", rather than used stand alone.

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edited May 14 '09 at 4:18
answered May 13 '09 at 20:42

Will Hartung
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What is the difference between a mixin and inheritance?
A mix-in is a base class you can inherit from to provide additional functionality. The name "mix-in" indicates it is intended to be mixed in with other code. As such, the inference is that you would not instantiate the mix-in class on its own. Frequently the mix-in is used with other base classes. Therefore mixins are a subset, or special case, of inheritance.

The advantages of using a mix-in over single inheritance are that you can write code for the functionality one time, and then use the same functionality in multiple different classes. The disadvantage is that you may need to look for that functionality in other places than where it is used, so it is good to mitigate that disadvantage by keeping it close by.

I have personally found a mix-in necessary to use over single inheritance where we are unittesting a lot of similar code, but the test-cases are instantiated based on their inheritance of a base case, and the only way to keep the code close at hand (and in the same module), without messing with coverage numbers, is to inherit from object, and have the child cases inherit from both the universal test-case base and the custom base that only applies to them.

Mixins in Comparison and Contrast with Abstract Base Classes
Both are a form of parent class that is not intended to be instantiated.

A mixin provides functionality, but is unable to directly use it. A user is intended to use it through a (sub)class.

An abstract base class provides an interface, but without usable functionality. A user is intended to create the functionality called by the interface.

In Python, some classes in the abc module are examples of parent classes that both provide functionality through inheritance and abstract interfaces that must be implemented by the subclass. These ideas are not mutually exclusive.