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Can python class variable be used as both class and instance variable? [duplicate]

I'm simultaneously learning Python while picking up Django. I'm familiar with many other languages.
In the following code snippet, x is a class variable of class Foo.
class Foo(object):
x = 9000
Given the previous declaration, the following works fine.
print Foo.x
The Django framework lets you create your model by defining Python classes. It makes fields out of the different class variables in your Python classes.
class Question(models.Model):
question_text = models.CharField(max_length=200)
Why does the following code snippet:
#!/usr/bin/env
import os, django
os.environ['DJANGO_SETTINGS_MODULE'] = 'mysite.settings'
django.setup()
from polls.models import Question, Choice
print Question.question_text
throw the following error:
AttributeError: type object 'Question' has no attribute 'question_text'
As far as I'm understanding everything my Question class has a single static member defined: Question.question_text.

Django models use a metaclass to alter what is normal class behaviour.
Use dir(Question) and you'll see there are different attributes on that class now. This is custom behaviour just for Django models however.
If you are curious you can study the metaclass __new__ method, but it does a lot of work specific to Object Relational Mapping tasks.

Magic.
No, really.
Python classes aren't set-in-stone structure, like they are in C++. They are, themselves, just objects — instances of another type:
class Foo(object):
pass
print(type(Foo)) # <class 'type'>
You can even make a class like you'd make any other object, by calling type. This:
class Bar(object):
a = 1
b = 2
Is really (more or less) syntactic sugar for this:
Bar = type('Bar', (object,), {'a': 1, 'b': 2})
type takes the name of your new class, a list of its superclasses, and a dict of all the attributes of the class, and spits out a new class.
But, because type is just a class like any other, it's possible to subclass it and give it different behavior. And this is what Django has done: it's created a subclass of type that does something different with the dict of attributes you pass to it.
You don't see this happening directly in your own code, but if you check type(models.Model), you'll find out its type is not type, but something specific to Django. It probably has "meta" in the name, because it's called a metaclass: the class of a class.
This is a fairly common pattern for making "declarative" libraries in Python, where the attributes of a class actually define some kind of structure. You can see the same thing in form validation (wtforms), schema validation (colander), other ORMs (sqlalchemy), and even the stdlib enum module.

Question is an object of type type. You want an instance of Question:
>>> q= Question(text = "Does a dog have the buddha nature?")
Then you should get
q.text
"Does a dog have the buddha nature?"
Note that this object will not persist unless you save() it:
>>> q.save()

python - how to get a list of inner classes?

I'm writing a small Python application that contains a few nested classes, like the example below:
class SuperBar(object):
pass
class Foo(object):
NAME = 'this is foo'
class Bar(SuperBar):
MSG = 'this is how Bar handle stuff'
class AnotherBar(SuperBar):
MSG = 'this is how Another Bar handle stuff'
I'm using nested classes to create some sort of hierarchy and to provide a clean way to implement features for a parser.
At some point, I want to create a list of the inner classes. I'd like to have the following output:
[<class '__main__.Bar'>, <class '__main__.AnotherBar'>]
The question is: What is the recommended method to get a list of inner classes in a pythonic way?

I managed to get a list of inner class objects with the method below:
import inspect
def inner_classes_list(cls):
return [cls_attribute for cls_attribute in cls.__dict__.values()
if inspect.isclass(cls_attribute)
and issubclass(cls_attribute, SuperBar)]
It works, but I'm not sure if using __dict__ directly is a good thing to do. I'm using it because it contains the actual class instances that I need and seems to be portable across Python 2 and 3.

First: I can't see how nested classes can be of any use for you. Once you have an instance f of Foo, do you realize that f.Bar and f.AnotherBar will be the same object for all instances? That is - you can't record any attribute specific from f on f.Bar, like f.Bar.speed - or it will collide with an attribute from another instance g.Bar.speed.
To overcome this, and actually, the only thing that makes sense, you'd need to have instances of Bar and AnotherBar attached to the instance f. These instances usually can't be declared on the class body - you have to create them on your Foo's __init__ method.
The only thing that Bar and AntherBar can do doing there is: (1) to have a lot of class and static methods, then they work as namespaces only.
Or, if a metaclass for SuperBar or themselves implement the descriptor protocol - https://docs.python.org/3/reference/datamodel.html#implementing-descriptors - but them, you'd be much better if superbar itself would implement the descriptor prootocol (by having either __get__ or __set__ methods), and attached to Foo's body you'd have instances of these classes, not the classes themselves.
That said, you came with the solution of using __dict__ to getting the inner classes: that won't work if Foo itself inherit from other classes that also have nested classes. The Superclasses of Foo are never searched. You can have a method to either look on all classes on Foo's __mro__, or simply use dir and issubclass :
class Foo:
#classmethod
def inner_classes_list(cls):
results = []
for attrname in dir(cls):
obj = getattr(cls, attrname)
if isinstance(obj, type) and issubclass(obj, SuperBar):
results.append(obj)
return results
(If you want this to work to all classes like Foo that does not share a common base, the same code will work if it is nto declared as a class method, of course - and also, SuperBar can be a parameter to this function, if you have more than one nested-class hierarchy.)
Now you have this, we urge you to ask other questions saying what do you want to actually do - and to read about "descriptors" - and even "properties". Really: there is very little use one can think of to nested subclasses.

Python: Pickle derived classes as if they were an instance of the base class

I want to define a base class so that when derived class instances are pickled, they are pickled as if they are instances of the base class. This is because the derived classes may exist on the client side of the pickling but not on the server side, but this is not important to the server since it only needs information from the base class. I don't want to have to dynamically create classes for every client.
The base class is simply an "object handle" which contains an ID, with methods defined on the server, but I would like the client to be able to subclass the server classes and define new methods (which would only be seen by the client, but that doesn't matter).

I believe you can do it by giving the object a __reduce__ method, returning a tuple, the first part of which should be BaseClass.__new__ (this will be called when loading the object in unpickling). See the pickle documentation (Python 2, Python 3) for the full details. I haven't attempted this.
Depending on what you're doing, it might be easier to use a simpler serialisation format like JSON, and have code on each side to reconstruct the relevant objects.

You can change an object's class dynamically in Python:
import cPickle as pickle
class Foo(object):
def __init__(self):
self.id=1
class Bar(Foo):
def derived_class_method(self): pass
bar=Bar()
bar.id=2
bar.__class__=Foo # changes `bar`'s class to Foo
bar_pickled=pickle.dumps(bar)
bar2=pickle.loads(bar_pickled)
bar.__class__=Bar # reset `bar`'s class to Bar
print(repr(bar2))
# <__main__.Foo object at 0xb76b08ec>
print(bar2.id)
# 2
I'm not sure using this is the best design decision, however. I like Thomas K's idea of using JSON.

Multiple inheritance in django. Problem with constructors

I have a model like this:
class Person(models.Model,Subject):
name = ..
The class Subject is not supposed to be in the Database so, it doesn't extends from models.Model:
class Subject:
def __init__(self,**kargs):
_observers = []
my problem is that the constructor of Subject is never called, so i've tried adding this to the class Person:
def __init__(self):
super(Person,self).__init__()
but now i have an error saying that init takes 1 arguments but 7 are given, and the only thing i'm doing is
>>> Person.objects.get(pk=1)
now i'm lost =S do you have any idea how the constructor of person should be?
BTW: i'm using django 1.1 and python 2.6

First of all, use new-style classes (ones that inherit from object). Second, read about how python's super behaves in multiple inheritance scenarios: http://fuhm.net/super-harmful/
There is also a nice talk covering it: http://europythonvideos.blip.tv/file/4000758/

You can use Django's post_init signal. It's invoked after the model is instantiated, and is passed the instance that was created.

Need help with the class and instance concept in Python

I have read several documentation already but the definition of "class" and "instance" didnt get really clear for me yet.
Looks like that "class" is like a combination of functions or methods that return some result is that correct? And how about the instance? I read that you work with the class you creat trough the instance but wouldnt be easier to just work direct with the class?
Sometimes geting the concepts of the language is harder than working with it.

Your question is really rather broad as classes and instances/objects are vital parts of object-oriented programming, so this is not really Python specific. I recommend you buy some books on this as, while initially basic, it can get pretty in-depth. In essense, however:
The most popular and developed model of OOP is a class-based model, as opposed to an object-based model. In this model, objects are entities that combine state (i.e., data), behavior (i.e., procedures, or methods) and identity (unique existence among all other objects). The structure and behavior of an object are defined by a class, which is a definition, or blueprint, of all objects of a specific type. An object must be explicitly created based on a class and an object thus created is considered to be an instance of that class. An object is similar to a structure, with the addition of method pointers, member access control, and an implicit data member which locates instances of the class (i.e. actual objects of that class) in the class hierarchy (essential for runtime inheritance features).
So you would, for example, define a Dog class, and create instances of particular dogs:
>>> class Dog():
... def __init__(self, name, breed):
... self.name = name
... self.breed = breed
... def talk(self):
... print "Hi, my name is " + self.name + ", I am a " + self.breed
...
>>> skip = Dog('Skip','Bulldog')
>>> spot = Dog('Spot','Dalmatian')
>>> spot.talk()
Hi, my name is Spot, I am a Dalmatian
>>> skip.talk()
Hi, my name is Skip, I am a Bulldog
While this example is silly, you can then start seeing how you might define a Client class that sets a blueprint for what a Client is, has methods to perform actions on a particular client, then manipulate a particular instance of a client by creating an object and calling these methods in that context.
Sometimes, however, you have methods of a class that don't really make sense being accessed through an instance of the class, but more from the class itself. These are known as static methods.

I am not sure of what level of knowledge you have, so I apologize if this answer is too simplified (then just ignore it).
A class is a template for an object. Like a blueprint for a car. The instance of a class is like an actual car. So you have one blueprint, but you can have several different instances of cars. The blueprint and the car are different things.
So you make a class that describes what an instance of that class can do and what properties it should have. Then you "build" the instance and get an object that you can work with.

It's fairly simple actually. You know how in python they say "everything is an object". Well in simplistic terms you can think of any object as being an 'instance' and the instructions to create an object as the class. Or in biological terms DNA is the class and you are an instance of DNA.
class HumanDNA(): # class
... class attributes ...
you = HumanDNA() # instance

See http://homepage.mac.com/s_lott/books/python/htmlchunks/ch21.html
Object-oriented programming permits us
to organize our programs around the
interactions of objects. A class
provides the definition of the
structure and behavior of the objects;
each object is an instance of a class.
Objects ("instances") are things which interact, do work, persist in the file system, etc.
Classes are the definitions for the object's behavior.
Also, a class creates new objects that are members of that class (share common structure and behavior)

In part it is confusing due to the dynamically typed nature of Python, which allows you to operate on a class and an instance in essentially the same way. In other languages, the difference is more concrete in that a class provides a template by which to create an object (instance) and cannot be as directly manipulated as in Python. The benefit of operating on the instance rather than the class is that the class can provide a prototype upon which instances are created.