Develop Reference

Get PyCharm to know what classes are mixin for

Our application has set of complex form wizards. To avoid code duplication I created several mixins.
The problem is that PyCharm highlights mixin methods with Unresolved attribute refference error.
This is correct as object does not have such methods. But I know that this mixin will be used only with special classes. Is there any way to tell this info to PyCharm?
For now I use such approach:
class MyMixin(object):
def get_context_data(self, **kwargs):
assert isinstance(self, (ClassToBeExtended, MyMixin))
# super.get_context_data is still highlighter,
# as super is considered as object
context = super(MyMixin, self).get_context_data(**kwargs)
context.update(self.get_preview_context())
return context
def get_preview_context(self):
# without this line PyCharm highlights the self.initial_data
assert isinstance(self, (ClassToBeExtended, MyMixin))
return {'needs': (self.initial_data['needs']
if 'type' not in self.initial_data
else '%(needs)s %(type)s' % self.initial_data)}
While this works for some cases like autocomplete for self., it fails for other cases like super. Is there a better approach to achieve the desired behavior?
P.S.: I know that I can disable reference check for specific name or whole class, but I don't want to do this as it will not help in typo checks and autocomplete.

You can type-hint to PyCharm what kind of classes to expect.
class DictMixin(object):
def megamethod(
self, # type: dict
key
):
return self.get(key)
It's still not quite comparable to other type handling.
PyCharm is lazy in evaluating it, and only does so when first working on self.
Things are a bit tricky when accessing attributes of the mixin as well - self, # type: dict | DictMixin works for one of my classes, but not in my test code.
In python 3.5, you should be able to use # type: typing.Union[dict, DictMixin].

If you are creating Mixin, for, let's say ClassSub, which is subclass of ClassSuper, you can implement Mixins this way:
class Mixin1(ClassSuper):
pass
class Mixin2(ClassSuper):
pass
and then use them like:
class ClassSub(Mixin1, Mixin2):
pass
That way I use some mixins for models in Django. Also, django-extensions uses similar pattern (gives models that are actually mixins). Basically, this way you don't have to inherit ClassSuper, because it's "included" in every of your mixins.
Most important - PyCharm works like a charm this way.

Subclasses vs Mixins in Python [duplicate]

In Programming Python, Mark Lutz mentions the term mixin. I am 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 have linked to because it is quite long), I am presuming it is 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?

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.

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. :)

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 "authoritative" 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 <= and ==:
class ComparableMixin(object):
"""This class has methods which use `<=` and `==`,
but this class does NOT implement those methods."""
def __ne__(self, other):
return not (self == other)
def __lt__(self, other):
return self <= other and (self != other)
def __gt__(self, other):
return not self <= other
def __ge__(self, other):
return self == other or self > other
class Integer(ComparableMixin):
def __init__(self, i):
self.i = i
def __le__(self, other):
return self.i <= other.i
def __eq__(self, other):
return self.i == other.i
assert Integer(0) < Integer(1)
assert Integer(0) != Integer(1)
assert Integer(1) > Integer(0)
assert Integer(1) >= 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 <= other.i
def __eq__(self, other):
return self.i == other.i
assert Integer(0) < Integer(1)
assert Integer(0) != Integer(1)
assert Integer(1) > Integer(0)
assert Integer(1) >= 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

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.

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:
>>> f1 = Foo('bar')
>>> f2 = Foo()
>>> f1
Foo(foo='bar')
>>> f2
Foo(foo=None)

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.

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

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.

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.

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.

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 <iostream>
#include <assert.h>
template <class T>
class ComparableMixin {
public:
bool operator !=(ComparableMixin &other) {
return ~(*static_cast<T*>(this) == static_cast<T&>(other));
}
bool operator <(ComparableMixin &other) {
return ((*(this) != other) && (*static_cast<T*>(this) <= static_cast<T&>(other)));
}
bool operator >(ComparableMixin &other) {
return ~(*static_cast<T*>(this) <= static_cast<T&>(other));
}
bool operator >=(ComparableMixin &other) {
return ((*static_cast<T*>(this) == static_cast<T&>(other)) || (*(this) > other));
}
protected:
ComparableMixin() {}
};
class Integer: public ComparableMixin<Integer> {
public:
Integer(int i) {
this->i = i;
}
int i;
bool operator <=(Integer &other) {
return (this->i <= other.i);
}
bool operator ==(Integer &other) {
return (this->i == other.i);
}
};
int main() {
Integer i(0) ;
Integer j(1) ;
//ComparableMixin<Integer> c; // this will cause compilation error because constructor is protected.
assert (i < j );
assert (i != j);
assert (j > i);
assert (j >= 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.

The concept comes from Steve’s Ice Cream, an ice cream store founded by Steve Herrell in Somerville, Massachusetts, in 1973, where mix-ins (candies, cakes, etc.) were mixed into basic ice cream flavors (vanilla, chocolate, etc.).
Inspired by Steve’s Ice Cream, the designers of the Lisp object system Flavors included the concept in a programming language for the first time, where mix-ins were small helper classes designed for enhancing other classes, and flavors were large standalone classes.
So the main idea is that a mix-in is a reusable extension (’reusable’ as opposed to ‘exclusive’; ‘extension’ as opposed to ‘base’).
The concept is orthogonal to the concepts of single or multiple inheritance and abstract or concrete class. Mix-in classes can be used in single or multiple inheritance and can be abstract or concrete classes. Mix-in classes have incomplete interfaces while abstract classes have incomplete implementations and concrete classes have complete implementations.
Mix-in class names are conventionally suffixed with ‘-MixIn’, ‘-able’, or ‘-ible’ to emphasize their nature, like in the Python standard library with the ThreadingMixIn and ForkingMixIn classes of the socketserver module, and the Hashable, Iterable, Callable, Awaitable, AsyncIterable, and Reversible classes of the collections.abc module.
Here is an example of a mix-in class used for extending the Python built-in list and dict classes with logging capability:
import logging
class LoggingMixIn:
def __setitem__(self, key, value):
logging.info('Setting %r to %r', key, value)
super().__setitem__(key, value)
def __delitem__(self, key):
logging.info('Deleting %r', key)
super().__delitem__(key)
class LoggingList(LoggingMixIn, list):
pass
class LoggingDict(LoggingMixIn, dict):
pass
>>> logging.basicConfig(level=logging.INFO)
>>> l = LoggingList([False])
>>> d = LoggingDict({'a': False})
>>> l[0] = True
INFO:root:Setting 0 to True
>>> d['a'] = True
INFO:root:Setting 'a' to True
>>> del l[0]
INFO:root:Deleting 0
>>> del d['a']
INFO:root:Deleting 'a'

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

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&view=markup

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.

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

Roughly summarizing all great answers above:
                States        /     Methods
Concrete Method
Abstract Method
Concrete State
Class
Abstract Class
Abstract State
Mixin
Interface

In Python, when should I use a meta class?

I have gone through this: What is a metaclass in Python?
But can any one explain more specifically when should I use the meta class concept and when it's very handy?
Suppose I have a class like below:
class Book(object):
CATEGORIES = ['programming','literature','physics']
def _get_book_name(self,book):
return book['title']
def _get_category(self, book):
for cat in self.CATEGORIES:
if book['title'].find(cat) > -1:
return cat
return "Other"
if __name__ == '__main__':
b = Book()
dummy_book = {'title':'Python Guide of Programming', 'status':'available'}
print b._get_category(dummy_book)
For this class.
In which situation should I use a meta class and why is it useful?
Thanks in advance.

You use metaclasses when you want to mutate the class as it is being created. Metaclasses are hardly ever needed, they're hard to debug, and they're difficult to understand -- but occasionally they can make frameworks easier to use. In our 600Kloc code base we've used metaclasses 7 times: ABCMeta once, 4x models.SubfieldBase from Django, and twice a metaclass that makes classes usable as views in Django. As #Ignacio writes, if you don't know that you need a metaclass (and have considered all other options), you don't need a metaclass.

Conceptually, a class exists to define what a set of objects (the instances of the class) have in common. That's all. It allows you to think about the instances of the class according to that shared pattern defined by the class. If every object was different, we wouldn't bother using classes, we'd just use dictionaries.
A metaclass is an ordinary class, and it exists for the same reason; to define what is common to its instances. The default metaclass type provides all the normal rules that make classes and instances work the way you're used to, such as:
Attribute lookup on an instance checks the instance followed by its class, followed by all superclasses in MRO order
Calling MyClass(*args, **kwargs) invokes i = MyClass.__new__(MyClass, *args, **kwargs) to get an instance, then invokes i.__init__(*args, **kwargs) to initialise it
A class is created from the definitions in a class block by making all the names bound in the class block into attributes of the class
Etc
If you want to have some classes that work differently to normal classes, you can define a metaclass and make your unusual classes instances of the metaclass rather than type. Your metaclass will almost certainly be a subclass of type, because you probably don't want to make your different kind of class completely different; just as you might want to have some sub-set of Books behave a bit differently (say, books that are compilations of other works) and use a subclass of Book rather than a completely different class.
If you're not trying to define a way of making some classes work differently to normal classes, then a metaclass is probably not the most appropriate solution. Note that the "classes define how their instances work" is already a very flexible and abstract paradigm; most of the time you do not need to change how classes work.
If you google around, you'll see a lot of examples of metaclasses that are really just being used to go do a bunch of stuff around class creation; often automatically processing the class attributes, or finding new ones automatically from somewhere. I wouldn't really call those great uses of metaclasses. They're not changing how classes work, they're just processing some classes. A factory function to create the classes, or a class method that you invoke immediately after class creation, or best of all a class decorator, would be a better way to implement this sort of thing, in my opinion.
But occasionally you find yourself writing complex code to get Python's default behaviour of classes to do something conceptually simple, and it actually helps to step "further out" and implement it at the metaclass level.
A fairly trivial example is the "singleton pattern", where you have a class of which there can only be one instance; calling the class will return an existing instance if one has already been created. Personally I am against singletons and would not advise their use (I think they're just global variables, cunningly disguised to look like newly created instances in order to be even more likely to cause subtle bugs). But people use them, and there are huge numbers of recipes for making singleton classes using __new__ and __init__. Doing it this way can be a little irritating, mainly because Python wants to call __new__ and then call __init__ on the result of that, so you have to find a way of not having your initialisation code re-run every time someone requests access to the singleton. But wouldn't be easier if we could just tell Python directly what we want to happen when we call the class, rather than trying to set up the things that Python wants to do so that they happen to do what we want in the end?
class Singleton(type):
def __init__(self, *args, **kwargs):
super(Singleton, self).__init__(*args, **kwargs)
self.__instance = None
def __call__(self, *args, **kwargs):
if self.__instance is None:
self.__instance = super(Singleton, self).__call__(*args, **kwargs)
return self.__instance
Under 10 lines, and it turns normal classes into singletons simply by adding __metaclass__ = Singleton, i.e. nothing more than a declaration that they are a singleton. It's just easier to implement this sort of thing at this level, than to hack something out at the class level directly.
But for your specific Book class, it doesn't look like you have any need to do anything that would be helped by a metaclass. You really don't need to reach for metaclasses unless you find the normal rules of how classes work are preventing you from doing something that should be simple in a simple way (which is different from "man, I wish I didn't have to type so much for all these classes, I wonder if I could auto-generate the common bits?"). In fact, I have never actually used a metaclass for something real, despite using Python every day at work; all my metaclasses have been toy examples like the above Singleton or else just silly exploration.

A metaclass is used whenever you need to override the default behavior for classes, including their creation.
A class gets created from the name, a tuple of bases, and a class dict. You can intercept the creation process to make changes to any of those inputs.
You can also override any of the services provided by classes:
__call__ which is used to create instances
__getattribute__ which is used to lookup attributes and methods on a class
__setattr__ which controls setting attributes
__repr__ which controls how the class is diplayed
In summary, metaclasses are used when you need to control how classes are created or when you need to alter any of the services provided by classes.

If you for whatever reason want to do stuff like Class[x], x in Class etc., you have to use metaclasses:
class Meta(type):
def __getitem__(cls, x):
return x ** 2
def __contains__(cls, x):
return int(x ** (0.5)) == x ** 0.5
# Python 2.x
class Class(object):
__metaclass__ = Meta
# Python 3.x
class Class(metaclass=Meta):
pass
print Class[2]
print 4 in Class

check the link Meta Class Made Easy to know how and when to use meta class.

How do I memoize expensive calculations on Django model objects?

I have several TextField columns on my UserProfile object which contain JSON objects. I've also defined a setter/getter property for each column which encapsulates the logic for serializing and deserializing the JSON into python datastructures.
The nature of this data ensures that it will be accessed many times by view and template logic within a single Request. To save on deserialization costs, I would like to memoize the python datastructures on read, invalidating on direct write to the property or save signal from the model object.
Where/How do I store the memo? I'm nervous about using instance variables, as I don't understand the magic behind how any particular UserProfile is instantiated by a query. Is __init__ safe to use, or do I need to check the existence of the memo attribute via hasattr() at each read?
Here's an example of my current implementation:
class UserProfile(Model):
text_json = models.TextField(default=text_defaults)
#property
def text(self):
if not hasattr(self, "text_memo"):
self.text_memo = None
self.text_memo = self.text_memo or simplejson.loads(self.text_json)
return self.text_memo
#text.setter
def text(self, value=None):
self.text_memo = None
self.text_json = simplejson.dumps(value)

You may be interested in a built-in django decorator django.utils.functional.memoize.
Django uses this to cache expensive operation like url resolving.

Generally, I use a pattern like this:
def get_expensive_operation(self):
if not hasattr(self, '_expensive_operation'):
self._expensive_operation = self.expensive_operation()
return self._expensive_operation
Then you use the get_expensive_operation method to access the data.
However, in your particular case, I think you are approaching this in slightly the wrong way. You need to do the deserialization when the model is first loaded from the database, and serialize on save only. Then you can simply access the attributes as a standard Python dictionary each time. You can do this by defining a custom JSONField type, subclassing models.TextField, which overrides to_python and get_db_prep_save.
In fact someone's already done it: see here.

For class methods, you should use django.utils.functional.cached_property.
Since the first argument on a class method is self, memoize will maintain a reference to the object and the results of the function even after you've thrown it away. This can cause memory leaks by preventing the garbage collector from cleaning up the stale object. cached_property turns Daniel's suggestion into a decorator.

Python metaclasses

I've been hacking classes in Python like this:
def hack(f,aClass) :
class MyClass(aClass) :
def f(self) :
f()
return MyClass
A = hack(afunc,A)
Which looks pretty clean to me. It takes a class, A, creates a new class derived from it that has an extra method, calling f, and then reassigns the new class to A.
How does this differ from metaclass hacking in Python? What are the advantages of using a metaclass over this?

The definition of a class in Python is an instance of type (or an instance of a subclass of type). In other words, the class definition itself is an object. With metaclasses, you have the ability to control the type instance that becomes the class definition.
When a metaclass is invoked, you have the ability to completely re-write the class definition. You have access to all the proposed attributes of the class, its ancestors, etc. More than just injecting a method or removing a method, you can radically alter the inheritance tree, the type, and pretty much any other aspect. You can also chain metaclasses together for a very dynamic and totally convoluted experience.
I suppose the real benefit, though is that the class's type remains the class's type. In your example, typing:
a_inst = A()
type(a_inst)
will show that it is an instance of MyClass. Yes, isinstance(a_inst, aClass) would return True, but you've introduced a subclass, rather than a dynamically re-defined class. The distinction there is probably the key.
As rjh points out, the anonymous inner class also has performance and extensibility implications. A metaclass is processed only once, and the moment that the class is defined, and never again. Users of your API can also extend your metaclass because it is not enclosed within a function, so you gain a certain degree of extensibility.
This slightly old article actually has a good explanation that compares exactly the "function decoration" approach you used in the example with metaclasses, and shows the history of the Python metaclass evolution in that context: http://www.ibm.com/developerworks/linux/library/l-pymeta.html

You can use the type callable as well.
def hack(f, aClass):
newfunc = lambda self: f()
return type('MyClass', (aClass,), {'f': newfunc})
I find using type the easiest way to get into the metaclass world.

A metaclass is the class of a class. IMO, the bloke here covered it quite serviceably, including some use-cases. See Stack Overflow question "MetaClass", "new", "cls" and "super" - what is the mechanism exactly?.