[python] What is a mixin, and why are they useful?
First, you should note that mixins only exist in multiple-inheritance languages. You can't do a mixin in Java or C#.
Basically, a mixin is a stand-alone base type that provides limited functionality and polymorphic resonance for a child class. If you're thinking in C#, think of an interface that you don't have to actually implement because it's already implemented; you just inherit from it and benefit from its functionality.
Mixins are typically narrow in scope and not meant to be extended.
[edit -- as to why:]
I suppose I should address why, since you asked. The big benefit is that you don't have to do it yourself over and over again. In C#, the biggest place where a mixin could benefit might be from the Disposal pattern. Whenever you implement IDisposable, you almost always want to follow the same pattern, but you end up writing and re-writing the same basic code with minor variations. If there were an extendable Disposal mixin, you could save yourself a lot of extra typing.
[edit 2 -- to answer your other questions]
What separates a mixin from multiple inheritance? Is it just a matter of semantics?
Yes. The difference between a mixin and standard multiple inheritance is just a matter of semantics; a class that has multiple inheritance might utilize a mixin as part of that multiple inheritance.
The point of a mixin is to create a type that can be "mixed in" to any other type via inheritance without affecting the inheriting type while still offering some beneficial functionality for that type.
Again, think of an interface that is already implemented.
I personally don't use mixins since I develop primarily in a language that doesn't support them, so I'm having a really difficult time coming up with a decent example that will just supply that "ahah!" moment for you. But I'll try again. I'm going to use an example that's contrived -- most languages already provide the feature in some way or another -- but that will, hopefully, explain how mixins are supposed to be created and used. Here goes:
Suppose you have a type that you want to be able to serialize to and from XML. You want the type to provide a "ToXML" method that returns a string containing an XML fragment with the data values of the type, and a "FromXML" that allows the type to reconstruct its data values from an XML fragment in a string. Again, this is a contrived example, so perhaps you use a file stream, or an XML Writer class from your language's runtime library... whatever. The point is that you want to serialize your object to XML and get a new object back from XML.
The other important point in this example is that you want to do this in a generic way. You don't want to have to implement a "ToXML" and "FromXML" method for every type that you want to serialize, you want some generic means of ensuring that your type will do this and it just works. You want code reuse.
If your language supported it, you could create the XmlSerializable mixin to do your work for you. This type would implement the ToXML and the FromXML methods. It would, using some mechanism that's not important to the example, be capable of gathering all the necessary data from any type that it's mixed in with to build the XML fragment returned by ToXML and it would be equally capable of restoring that data when FromXML is called.
And.. that's it. To use it, you would have any type that needs to be serialized to XML inherit from XmlSerializable. Whenever you needed to serialize or deserialize that type, you would simply call ToXML or FromXML. In fact, since XmlSerializable is a fully-fledged type and polymorphic, you could conceivably build a document serializer that doesn't know anything about your original type, accepting only, say, an array of XmlSerializable types.
Now imagine using this scenario for other things, like creating a mixin that ensures that every class that mixes it in logs every method call, or a mixin that provides transactionality to the type that mixes it in. The list can go on and on.
If you just think of a mixin as a small base type designed to add a small amount of functionality to a type without otherwise affecting that type, then you're golden.
In "Programming Python", Mark Lutz mentions "mixins". I'm from a C/C++/C# background and I have not heard the term before. What is a mixin?
Reading between the lines of this example (which I've linked to because it's quite long), I'm presuming it's a case of using multiple inheritance to extend a class as opposed to 'proper' subclassing. Is this right?
Why would I want to do that rather than put the new functionality into a subclass? For that matter, why would a mixin/multiple inheritance approach be better than using composition?
What separates a mixin from multiple inheritance? Is it just a matter of semantics?
I read that you have a c# background. So a good starting point might be a mixin implementation for .NET.
You might want to check out the codeplex project at http://remix.codeplex.com/
Watch the lang.net Symposium link to get an overview. There is still more to come on documentation on codeplex page.
I think of them as a disciplined way of using multiple inheritance - because ultimately a mixin is just another python class that (might) follow the conventions about classes that are called mixins.
My understanding of the conventions that govern something you would call a Mixin are that a Mixin:
- adds methods but not instance variables (class constants are OK)
- only inherits from
That way it limits the potential complexity of multiple inheritance, and makes it reasonably easy to track the flow of your program by limiting where you have to look (compared to full multiple inheritance). They are similar to ruby modules.
If I want to add instance variables (with more flexibility than allowed for by single inheritance) then I tend to go for composition.
Having said that, I have seen classes called XYZMixin that do have instance variables.
I think there have been some good explanations here but I wanted to provide another perspective.
In Scala, you can do mixins as has been described here but what is very interesting is that the mixins are actually 'fused' together to create a new kind of class to inherit from. In essence, you do not inherit from multiple classes/mixins, but rather, generate a new kind of class with all the properties of the mixin to inherit from. This makes sense since Scala is based on the JVM where multiple-inheritance is not currently supported (as of Java 8). This mixin class type, by the way, is a special type called a Trait in Scala.
It's hinted at in the way a class is defined: class NewClass extends FirstMixin with SecondMixin with ThirdMixin ...
I'm not sure if the CPython interpreter does the same (mixin class-composition) but I wouldn't be surprised. Also, coming from a C++ background, I would not call an ABC or 'interface' equivalent to a mixin -- it's a similar concept but divergent in use and implementation.
I'd advise against mix-ins in new Python code, if you can find any other way around it (such as composition-instead-of-inheritance, or just monkey-patching methods into your own classes) that isn't much more effort.
In old-style classes you could use mix-ins as a way of grabbing a few methods from another class. But in the new-style world everything, even the mix-in, inherits from
object. That means that any use of multiple inheritance naturally introduces MRO issues.
There are ways to make multiple-inheritance MRO work in Python, most notably the super() function, but it means you have to do your whole class hierarchy using super(), and it's considerably more difficult to understand the flow of control.
Maybe an example from ruby can help:
You can include the mixin
Comparable and define one function
"<=>(other)", the mixin provides all those functions:
<(other) >(other) ==(other) <=(other) >=(other) between?(other)
It does this by invoking
<=>(other) and giving back the right result.
"instance <=> other" returns 0 if both objects are equal, less than 0 if
instance is bigger than
other and more than 0 if
other is bigger.
mixin gives a way to add functionality in a class, i.e you can interact with methods defined in a module by including the module inside the desired class. Though ruby doesn't supports multiple inheritance but provides mixin as an alternative to achieve that.
here is an example that explains how multiple inheritance is achieved using mixin.
module A # you create a module def a1 # lets have a method 'a1' in it end def a2 # Another method 'a2' end end module B # let's say we have another module def b1 # A method 'b1' end def b2 #another method b2 end end class Sample # we create a class 'Sample' include A # including module 'A' in the class 'Sample' (mixin) include B # including module B as well def S1 #class 'Sample' contains a method 's1' end end samp = Sample.new # creating an instance object 'samp' # we can access methods from module A and B in our class(power of mixin) samp.a1 # accessing method 'a1' from module A samp.a2 # accessing method 'a2' from module A samp.b1 # accessing method 'b1' from module B samp.b2 # accessing method 'a2' from module B samp.s1 # accessing method 's1' inside the class Sample