What is a mixin and why are they useful

Question

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

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It s not a Python example but in the D programing language the term mixin is used to refer to a construct used much the same way  adding a pile of stuff to a class    In D  which by the way doesn t do MI  this is done by inserting a template  think syntactically aware and safe macros and you will be close  into a scope  This allows for a single line of code in a class  struct  function  module or whatever to expand to any number of declarations

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A mixin is a special kind of multiple inheritance   There are two main situations where mixins are used    You want to provide a lot of optional features for a class  You want to use one particular feature in a lot of different classes    For an example of number one  consider werkzeug s request and response system   I can make a plain old request object by saying   from werkzeug import BaseRequest  class Request BaseRequest       pass   If I want to add accept header support  I would make that  from werkzeug import BaseRequest  AcceptMixin  class Request AcceptMixin  BaseRequest       pass   If I wanted to make a request object that supports accept headers  etags  authentication  and user agent support  I could do this   from werkzeug import BaseRequest  AcceptMixin  ETagRequestMixin  UserAgentMixin  AuthenticationMixin  class Request AcceptMixin  ETagRequestMixin  UserAgentMixin  AuthenticationMixin  BaseRequest       pass   The difference is subtle  but in the above examples  the mixin classes weren t made to stand on their own   In more traditional multiple inheritance  the AuthenticationMixin  for example  would probably be something more like Authenticator   That is  the class would probably be designed to stand on its own

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I think previous responses defined very well what MixIns are  However   in order to better understand them  it might be useful to compare MixIns with Abstract Classes and Interfaces from the code implementation perspective   1  Abstract Class   Class that needs to contain one or more abstract methods Abstract Class can contain state  instance variables  and non-abstract methods   2  Interface    Interface contains abstract methods only  no non-abstract methods and no internal state    3  MixIns   MixIns  like Interfaces  do not contain internal state  instance variables  MixIns contain one or more non-abstract methods  they can contain non-abstract methods unlike interfaces    In e g  Python these are just conventions  because all of the above are defined as classes  However  the common feature of both Abstract Classes  Interfaces and MixIns is that they should not exist on their own  i e  should not be instantiated

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Maybe an example from ruby can help   You can include the mixin Comparable and define one function   lt   gt  other    the mixin provides all those functions    lt  other   gt  other     other   lt   other   gt   other  between  other    It does this by invoking  lt   gt  other  and giving back the right result    instance  lt   gt  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

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Perhaps a couple of examples will help   If you re building a class and you want it to act like a dictionary  you can define all the various       methods necessary   But that s a bit of a pain   As an alternative  you can just define a few  and inherit  in addition to any other inheritance  from UserDict DictMixin  moved to collections DictMixin in py3k    This will have the effect of automatically defining all the rest of the dictionary api   A second example  the GUI toolkit wxPython allows you to make list controls with multiple columns  like  say  the file display in Windows Explorer    By default  these lists are fairly basic   You can add additional functionality  such as the ability to sort the list by a particular column by clicking on the column header  by inheriting from ListCtrl and adding appropriate mixins

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What separates a mixin from multiple inheritance  Is it just a matter of semantics    A mixin is a limited form of multiple inheritance  In some languages the mechanism for adding a mixin to a class is slightly different  in terms of syntax  from that of inheritance   In the context of Python especially  a mixin is a parent class that provides functionality to subclasses but is not intended to be instantiated itself    What might cause you to say   that s just multiple inheritance  not really a mixin  is if the class that might be confused for a mixin can actually be instantiated and used - so indeed it is a semantic  and very real  difference   Example of Multiple Inheritance  This example  from the documentation  is an OrderedCounter    class OrderedCounter Counter  OrderedDict         Counter that remembers the order elements are first encountered        def   repr   self            return   s  r      self   class     name    OrderedDict self         def   reduce   self            return self   class     OrderedDict self       It subclasses both the Counter and the OrderedDict from the collections module   Both Counter and OrderedDict are intended to be instantiated and used on their own  However  by subclassing them both  we can have a counter that is ordered and reuses the code in each object    This is a powerful way to reuse code  but it can also be problematic  If it turns out there s a bug in one of the objects  fixing it without care could create a bug in the subclass    Example of a Mixin  Mixins are usually promoted as the way to get code reuse without potential coupling issues that cooperative multiple inheritance  like the OrderedCounter  could have  When you use mixins  you use functionality that isn t as tightly coupled to the data    Unlike the example above  a mixin is not intended to be used on its own  It provides new or different functionality   For example  the standard library has a couple of mixins in the socketserver library      Forking and threading versions of each type of server can be created   using these mix-in classes  For instance  ThreadingUDPServer is   created as follows   class ThreadingUDPServer ThreadingMixIn  UDPServer       pass       The mix-in class comes first  since it overrides a method defined in   UDPServer  Setting the various attributes also changes the behavior of   the underlying server mechanism    In this case  the mixin methods override the methods in the UDPServer object definition to allow for concurrency   The overridden method appears to be process request and it also provides another method  process request thread  Here it is from the source code    class ThreadingMixIn             Mix-in class to handle each request in a new thread                Decides how threads will act upon termination of the           main process         daemon threads   False          def process request thread self  request  client address                  Same as in BaseServer but as a thread              In addition  exception handling is done here                              try                  self finish request request  client address              except Exception                  self handle error request  client address              finally                  self shutdown request request           def process request self  request  client address                  Start a new thread to process the request                 t   threading Thread target   self process request thread                                   args    request  client address               t daemon   self daemon threads             t start      A Contrived Example  This is a mixin that is mostly for demonstration purposes - most objects will evolve beyond the usefulness of this repr   class SimpleInitReprMixin object          mixin  don t instantiate - useful for classes instantiable     by keyword arguments to their   init   method                slots          allow subclasses to use   slots   to prevent   dict       def   repr   self           kwarg strings              d   getattr self     dict     None          if d is not None              for k  v in d items                    kwarg strings append   k   v   format k k  v repr v            slots   getattr self     slots     None          if slots is not None              for k in slots                  v   getattr self  k  None                  kwarg strings append   k   v   format k k  v repr v            return   name   kwargs    format            name type self    name              kwargs      join kwarg strings                and usage would be   class Foo SimpleInitReprMixin     add other mixins and or extend another class here       slots      foo       def   init   self  foo None           self foo   foo         super Foo  self    init       And usage    gt  gt  gt  f1   Foo  bar    gt  gt  gt  f2   Foo    gt  gt  gt  f1 Foo foo  bar    gt  gt  gt  f2 Foo foo None

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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

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This answer aims to explain mixins with examples that are   self-contained  short  with no need to know any libraries to understand the example   in Python  not in other languages  It is understandable that there were examples from other languages such as Ruby since the term is much more common in those languages  but this is a Python thread    It shall also consider the controversial question   Is multiple inheritance necessary or not to characterize a mixin   Definitions I have yet to see a citation from an  quot authoritative quot  source clearly saying what is a mixin in Python  I have seen 2 possible definitions of a mixin  if they are to be considered as different from other similar concepts such as abstract base classes   and people don t entirely agree on which one is correct  The consensus may vary between different languages  Definition 1  no multiple inheritance A mixin is a class such that some method of the class uses a method which is not defined in the class  Therefore the class is not meant to be instantiated  but rather serve as a base class  Otherwise the instance would have methods that cannot be called without raising an exception  A constraint which some sources add is that the class may not contain data  only methods  but I don t see why this is necessary  In practice however  many useful mixins don t have any data  and base classes without data are simpler to use  A classic example  is the implementation of all comparison operators from only  lt   and     class ComparableMixin object        quot  quot  quot This class has methods which use   lt    and           but this class does NOT implement those methods  quot  quot  quot      def   ne   self  other           return not  self    other      def   lt   self  other           return self  lt   other and  self    other      def   gt   self  other           return not self  lt   other     def   ge   self  other           return self    other or self  gt  other  class Integer ComparableMixin       def   init   self  i           self i   i     def   le   self  other           return self i  lt   other i     def   eq   self  other           return self i    other i  assert Integer 0   lt   Integer 1  assert Integer 0     Integer 1  assert Integer 1   gt   Integer 0  assert Integer 1   gt   Integer 1     It is possible to instantiate a mixin  o   ComparableMixin     but one of its methods raise an exception   o    o   This particular example could have been achieved via the functools total ordering   decorator  but the game here was to reinvent the wheel  import functools   functools total ordering class Integer object       def   init   self  i           self i   i     def   le   self  other           return self i  lt   other i     def   eq   self  other           return self i    other i  assert Integer 0   lt  Integer 1  assert Integer 0     Integer 1  assert Integer 1   gt  Integer 0  assert Integer 1   gt   Integer 1   Definition 2  multiple inheritance A mixin is a design pattern in which some method of a base class uses a method it does not define  and that method is meant to be implemented by another base class  not by the derived like in Definition 1  The term mixin class refers to base classes which are intended to be used in that design pattern  TODO those that use the method  or those that implement it   It is not easy to decide if a given class is a mixin or not  the method could be just implemented on the derived class  in which case we re back to Definition 1  You have to consider the author s intentions  This pattern is interesting because it is possible to recombine functionalities with different choices of base classes  class HasMethod1 object       def method self           return 1  class HasMethod2 object       def method self           return 2  class UsesMethod10 object       def usesMethod self           return self method     10  class UsesMethod20 object       def usesMethod self           return self method     20  class C1 10 HasMethod1  UsesMethod10   pass class C1 20 HasMethod1  UsesMethod20   pass class C2 10 HasMethod2  UsesMethod10   pass class C2 20 HasMethod2  UsesMethod20   pass  assert C1 10   usesMethod      11 assert C1 20   usesMethod      21 assert C2 10   usesMethod      12 assert C2 20   usesMethod      22    Nothing prevents implementing the method   on the base class like in Definition 1   class C3 10 UsesMethod10       def method self           return 3  assert C3 10   usesMethod      13  Authoritative Python occurrences At the official documentatiton for collections abc the documentation explicitly uses the term Mixin Methods  It states that if a class   implements   next   inherits from a single class Iterator  then the class gets an   iter   mixin method for free  Therefore at least on this point of the documentation  mixin does not not require multiple inheritance  and is coherent with Definition 1  The documentation could of course be contradictory at different points  and other important Python libraries might be using the other definition in their documentation  This page also uses the term Set mixin  which clearly suggests that classes like Set and Iterator can be called Mixin classes  In other languages  Ruby  Clearly does not require multiple inheritance for mixin  as mentioned in major reference books such as Programming Ruby and The Ruby programming Language  C    A virtual method that is set  0 is a pure virtual method  Definition 1 coincides with the definition of an abstract class  a class that has a pure virtual method   That class cannot be instantiated  Definition 2 is possible with virtual inheritance  Multiple Inheritance from two derived classes

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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

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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 object  in Python    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

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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

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I just used a python mixin to implement unit testing for python milters   Normally  a milter talks to an MTA  making unit testing difficult   The test mixin overrides methods that talk to the MTA  and create a simulated environment driven by test cases instead   So  you take an unmodified milter application  like spfmilter  and mixin TestBase  like this   class TestMilter TestBase spfmilter spfMilter     def   init   self       TestBase   init   self      spfmilter config   spfmilter Config       spfmilter config access file    test access db      spfmilter spfMilter   init   self    Then  use TestMilter in the test cases for the milter application   def testPass self     milter   TestMilter     rc   milter connect  mail example com  ip  192 0 2 1     self assertEqual rc Milter CONTINUE    rc   milter feedMsg  test1  sender  good example com     self assertEqual rc Milter CONTINUE    milter close     http   pymilter cvs sourceforge net viewvc pymilter pymilter Milter test py revision 1 6 amp view markup

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Mixins is a concept in Programming in which the class provides functionalities but it is not meant to be used for instantiation  Main purpose of Mixins is to provide functionalities which are standalone and it would be best if the mixins itself do not have inheritance with other mixins and also avoid state  In languages such as Ruby  there is some direct language support but for Python  there isn t  However  you could used multi-class inheritance to execute the functionality provided in Python   I watched this video http   www youtube com watch v v uKI2NOLEM to understand the basics of mixins  It is quite useful for a beginner to understand the basics of mixins and how they work and the problems you might face in implementing them   Wikipedia is still the best  http   en wikipedia org wiki Mixin

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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    Hopefully

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OP mentioned that he she never heard of mixin in C    perhaps that is because they are called Curiously Recurring Template Pattern  CRTP  in C    Also   Ciro Santilli mentioned that mixin is implemented via abstract base class in C    While abstract base class can be used to implement mixin  it is an overkill as the functionality of virtual function at run-time can be achieved using template at compile time without the overhead of virtual table lookup at run-time    The CRTP pattern is described in detail here  I have converted the python example in  Ciro Santilli s answer into C   using template class below        include  lt iostream gt       include  lt assert h gt       template  lt class T gt      class ComparableMixin       public          bool operator    ComparableMixin  amp other                return    static cast lt T  gt  this     static cast lt T amp  gt  other                      bool operator  lt  ComparableMixin  amp other                return     this     other   amp  amp    static cast lt T  gt  this   lt   static cast lt T amp  gt  other                       bool operator  gt  ComparableMixin  amp other                return    static cast lt T  gt  this   lt   static cast lt T amp  gt  other                      bool operator  gt   ComparableMixin  amp other                return    static cast lt T  gt  this     static cast lt T amp  gt  other         this   gt  other                      protected              ComparableMixin                  class Integer  public ComparableMixin lt Integer gt        public       Integer int i             this- gt i   i              int i       bool operator  lt   Integer  amp other             return  this- gt i  lt   other i               bool operator    Integer  amp other             return  this- gt i    other i                  int main          Integer i 0        Integer j 1          ComparableMixin lt Integer gt  c     this will cause compilation error because constructor is protected      assert  i  lt  j        assert  i    j       assert  j  gt   i       assert  j  gt   i        return 0      EDIT  Added protected constructor in ComparableMixin so that it can only be inherited and not instantiated  Updated the example to show how protected constructor will cause compilation error when an object of ComparableMixin is created

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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   regards Stefan

[python] What is a mixin, and why are they useful?

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