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Extending parent/children classes in python

Is it possible in python for a nested class to extend its parent?
Like this:
class Parent:
class Child(Parent):
pass
child = Parent.Child()
Is it possible to do this in the opposite direction?
Like this:
class Parent(Child):
class Child:
pass
parent = Parent()
From what I know this is not possible, even with from __future__ import annotations.
The best known way around this is just not to make nested classes.
Important:
The purpose of this question is to make it clear if this is even possible in the python language.
There is no "final goal", objectives to be accomplished with this approach or justification for it.
Don't spam in the comments/answers about "how bad this code is".

No and Yes.
No, because when you inherit from a class, that class must be defined before you can inherit from it. Neither of your code examples will work due to this.
Yes, because Python is a dynamic language and you can change (monkey-patch) the base classes even after defining them:
class Temp:
pass
# example 1
class Parent:
class Child(Temp):
pass
Parent.Child.__bases__ = (Parent,)
# example 2
class Parent(Temp):
class Child:
pass
Parent.__bases__ = (Parent.Child,)
Why use the Temp class?
Classes automatically inherit from object. Due to a bug (https://bugs.python.org/issue672115), we cannot change __bases__ if a class inherits from object. Hence, we inherit from a temporary (Temp) class to avoid that issue.

How to do multiple inheritance from different classes in python using super()?

Lets say we have different kind of people, pianist,programmer and multitalented person.
so, How do i inherit like this? currently this code gives error Multitalented has no attribute canplaypiano.
class Pianist:
def __init__(self):
self.canplaypiano=True
class Programer:
def __init__(self):
self.canprogram=True
class Multitalented(Pianist,Programer):
def __init__(self):
self.canswim=True
super(Pianist,self).__init__()
super(Programer,self).__init__()
Raju=Multitalented()
print(Raju.canswim)
print(Raju.canprogram)
print(Raju.canplaypiano)
Also Please mention some well written article about python inheritance/super() i couldnt find a perfect article with clear explaination. thankyou.

All classes involved in cooperative multiple inheritance need to use super, even if the static base class is just object.
class Pianist:
def __init__(self):
super().__init__()
self.canplaypiano=True
class Programer:
def __init__(self):
super().__init__()
self.canprogram=True
class Multitalented(Pianist,Programer):
def __init__(self):
super().__init__()
self.canswim=True
Raju=Multitalented()
print(Raju.canswim)
print(Raju.canprogram)
print(Raju.canplaypiano)
The order in which the initializers run is determined by the method resolution order for Multitalented, which you can affect by changing the order in which Multitalented lists its base classes.
The first, if not best, article to read is Raymond Hettinger's Python's super() Considered Super!, which also includes advice on how to adapt classes the don't themselves use super for use in a cooperative multiple-inheritance hierarchy, as well as advice on how to override a function that uses super (in short, you can't change the signature).

Dont call super with explicit parent classes. In modern python versions (don't know exactly since which version) you call super without parameters. That is, in you case you should have had only one line, not two:
super().__init__()
In somewhat older versions you need to provide the class explicitly, however you should provide the class of "current" object, and the super function takes care of finding out the parent classes. In you case it should be:
super(Multitalented, self).__init__()

Should all Python classes extend object? [duplicate]

This question already has answers here:
Why do Python classes inherit object?
(6 answers)
Closed 1 year ago.
I have found that both of the following work:
class Foo():
def a(self):
print "hello"
class Foo(object):
def a(self):
print "hello"
Should all Python classes extend object? Are there any potential problems with not extending object?

In Python 2, not inheriting from object will create an old-style class, which, amongst other effects, causes type to give different results:
>>> class Foo: pass
...
>>> type(Foo())
<type 'instance'>
vs.
>>> class Bar(object): pass
...
>>> type(Bar())
<class '__main__.Bar'>
Also the rules for multiple inheritance are different in ways that I won't even try to summarize here. All good documentation that I've seen about MI describes new-style classes.
Finally, old-style classes have disappeared in Python 3, and inheritance from object has become implicit. So, always prefer new style classes unless you need backward compat with old software.

In Python 3, classes extend object implicitly, whether you say so yourself or not.
In Python 2, there's old-style and new-style classes. To signal a class is new-style, you have to inherit explicitly from object. If not, the old-style implementation is used.
You generally want a new-style class. Inherit from object explicitly. Note that this also applies to Python 3 code that aims to be compatible with Python 2.

In python 3 you can create a class in three different ways & internally they are all equal (see examples). It doesn't matter how you create a class, all classes in python 3 inherits from special class called object. The class object is fundamental class in python and provides lot of functionality like double-underscore methods, descriptors, super() method, property() method etc.
Example 1.
class MyClass:
pass
Example 2.
class MyClass():
pass
Example 3.
class MyClass(object):
pass

Yes, all Python classes should extend (or rather subclass, this is Python here) object. While normally no serious problems will occur, in some cases (as with multiple inheritance trees) this will be important. This also ensures better compatibility with Python 3.

As other answers have covered, Python 3 inheritance from object is implicit. But they do not state what you should do and what is convention.
The Python 3 documentation examples all use the following style which is convention, so I suggest you follow this for any future code in Python 3.
class Foo:
pass
Source: https://docs.python.org/3/tutorial/classes.html#class-objects
Example quote:
Class objects support two kinds of operations: attribute references
and instantiation.
Attribute references use the standard syntax used for all attribute
references in Python: obj.name. Valid attribute names are all the
names that were in the class’s namespace when the class object was
created. So, if the class definition looked like this:
class MyClass:
"""A simple example class"""
i = 12345
def f(self):
return 'hello world'
Another quote:
Generally speaking, instance variables are for data unique to each
instance and class variables are for attributes and methods shared by
all instances of the class:
class Dog:
kind = 'canine' # class variable shared by all instances
def __init__(self, name):
self.name = name # instance variable unique to each instance

in python3 there isn't a differance, but in python2 not extending object gives you an old-style classes; you'd like to use a new-style class over an old-style class.

Why do Python classes inherit object?

Why does the following class declaration inherit from object?
class MyClass(object):
...

Is there any reason for a class declaration to inherit from object?
In Python 3, apart from compatibility between Python 2 and 3, no reason. In Python 2, many reasons.
Python 2.x story:
In Python 2.x (from 2.2 onwards) there's two styles of classes depending on the presence or absence of object as a base-class:
"classic" style classes: they don't have object as a base class:
>>> class ClassicSpam: # no base class
... pass
>>> ClassicSpam.__bases__
()
"new" style classes: they have, directly or indirectly (e.g inherit from a built-in type), object as a base class:
>>> class NewSpam(object): # directly inherit from object
... pass
>>> NewSpam.__bases__
(<type 'object'>,)
>>> class IntSpam(int): # indirectly inherit from object...
... pass
>>> IntSpam.__bases__
(<type 'int'>,)
>>> IntSpam.__bases__[0].__bases__ # ... because int inherits from object
(<type 'object'>,)
Without a doubt, when writing a class you'll always want to go for new-style classes. The perks of doing so are numerous, to list some of them:
Support for descriptors. Specifically, the following constructs are made possible with descriptors:
classmethod: A method that receives the class as an implicit argument instead of the instance.
staticmethod: A method that does not receive the implicit argument self as a first argument.
properties with property: Create functions for managing the getting, setting and deleting of an attribute.
__slots__: Saves memory consumptions of a class and also results in faster attribute access. Of course, it does impose limitations.
The __new__ static method: lets you customize how new class instances are created.
Method resolution order (MRO): in what order the base classes of a class will be searched when trying to resolve which method to call.
Related to MRO, super calls. Also see, super() considered super.
If you don't inherit from object, forget these. A more exhaustive description of the previous bullet points along with other perks of "new" style classes can be found here.
One of the downsides of new-style classes is that the class itself is more memory demanding. Unless you're creating many class objects, though, I doubt this would be an issue and it's a negative sinking in a sea of positives.
Python 3.x story:
In Python 3, things are simplified. Only new-style classes exist (referred to plainly as classes) so, the only difference in adding object is requiring you to type in 8 more characters. This:
class ClassicSpam:
pass
is completely equivalent (apart from their name :-) to this:
class NewSpam(object):
pass
and to this:
class Spam():
pass
All have object in their __bases__.
>>> [object in cls.__bases__ for cls in {Spam, NewSpam, ClassicSpam}]
[True, True, True]
So, what should you do?
In Python 2: always inherit from object explicitly. Get the perks.
In Python 3: inherit from object if you are writing code that tries to be Python agnostic, that is, it needs to work both in Python 2 and in Python 3. Otherwise don't, it really makes no difference since Python inserts it for you behind the scenes.

Python 3
class MyClass(object): = New-style class
class MyClass: = New-style class (implicitly inherits from object)
Python 2
class MyClass(object): = New-style class
class MyClass: = OLD-STYLE CLASS
Explanation:
When defining base classes in Python 3.x, you’re allowed to drop the object from the definition. However, this can open the door for a seriously hard to track problem…
Python introduced new-style classes back in Python 2.2, and by now old-style classes are really quite old. Discussion of old-style classes is buried in the 2.x docs, and non-existent in the 3.x docs.
The problem is, the syntax for old-style classes in Python 2.x is the same as the alternative syntax for new-style classes in Python 3.x. Python 2.x is still very widely used (e.g. GAE, Web2Py), and any code (or coder) unwittingly bringing 3.x-style class definitions into 2.x code is going to end up with some seriously outdated base objects. And because old-style classes aren’t on anyone’s radar, they likely won’t know what hit them.
So just spell it out the long way and save some 2.x developer the tears.

Yes, this is a 'new style' object. It was a feature introduced in python2.2.
New style objects have a different object model to classic objects, and some things won't work properly with old style objects, for instance, super(), #property and descriptors. See this article for a good description of what a new style class is.
SO link for a description of the differences: What is the difference between old style and new style classes in Python?

History from Learn Python the Hard Way:
Python's original rendition of a class was broken in many serious
ways. By the time this fault was recognized it was already too late,
and they had to support it. In order to fix the problem, they needed
some "new class" style so that the "old classes" would keep working
but you can use the new more correct version.
They decided that they would use a word "object", lowercased, to be
the "class" that you inherit from to make a class. It is confusing,
but a class inherits from the class named "object" to make a class but
it's not an object really its a class, but don't forget to inherit
from object.
Also just to let you know what the difference between new-style classes and old-style classes is, it's that new-style classes always inherit from object class or from another class that inherited from object:
class NewStyle(object):
pass
Another example is:
class AnotherExampleOfNewStyle(NewStyle):
pass
While an old-style base class looks like this:
class OldStyle():
pass
And an old-style child class looks like this:
class OldStyleSubclass(OldStyle):
pass
You can see that an Old Style base class doesn't inherit from any other class, however, Old Style classes can, of course, inherit from one another. Inheriting from object guarantees that certain functionality is available in every Python class. New style classes were introduced in Python 2.2

Yes, it's historical. Without it, it creates an old-style class.
If you use type() on an old-style object, you just get "instance". On a new-style object you get its class.

The syntax of the class creation statement:
class <ClassName>(superclass):
#code follows
In the absence of any other superclasses that you specifically want to inherit from, the superclass should always be object, which is the root of all classes in Python.
object is technically the root of "new-style" classes in Python. But the new-style classes today are as good as being the only style of classes.
But, if you don't explicitly use the word object when creating classes, then as others mentioned, Python 3.x implicitly inherits from the object superclass. But I guess explicit is always better than implicit (hell)
Reference

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.