from abc import ABCMeta, abstractmethod
import random

class BaseSource:
    __metaclass__ = ABCMeta

    @abstractmethod
    def get(self):
        pass

class ManualSrc(BaseSource):
    def get(self):
        return float(raw_input('gimme a number - '))

class RandomSrc(BaseSource):
    def get(self):
        return random.random()

class Source(BaseSource):
    """generic Source choice"""
    def __new__(BaseSource, choice):
        if choice == 0:
            return ManualSrc()
        elif choice == 1:
            return RandomSrc()
        else:
            raise ValueError('source choice parameter {} not valid'.format(choice))

if __name__ == '__main__':
    for use_src in range(4):
        print 'using source choice {}'.format(use_src)
        src = Source(use_src)
        print src.get()

## [ . . . ]

class RandomSrc(BaseSource):
    def get(self):
        return random.random()

def create_source(choice):
    if choice == 0:
        return ManualSrc()
    elif choice == 1:
        return RandomSrc()
    else:
        raise ValueError('source choice parameter {} not valid'.format(choice))

if __name__ == '__main__':
    for use_src in range(4):
        print 'using source choice {}'.format(use_src)
        src = create_source(use_src)
        print src.get()

## [ . . . ]

class RandomSrc(BaseSource):
    def get(self):
        return random.random()

class WhateverSrc(BaseSource):
    def get(self):
        return "Foo Bar??"

def create_source(choice):
    if choice == 0:
        return ManualSrc()
    elif choice == 1:
        return RandomSrc()
    elif choice == 2:
        return WhateverSrc()
    else:
        raise ValueError('source choice parameter {} not valid'.format(choice))

import random

class ManualSrc(object):
    def get(self):
        return float(raw_input('gimme a number - '))

class RandomSrc(object):
    def get(self):
        return random.random()

class BreakingSrc(object):
    pass

def create_source(choice):
    if choice == 0:
        return ManualSrc()
    elif choice == 1:
        return RandomSrc()
    elif choice == 2:
        return BreakingSrc()
    else:
        raise ValueError('source choice parameter {} not valid'.format(choice))

if __name__ == '__main__':
    for use_src in range(4):
        print 'using source choice {}'.format(use_src)
        src = create_source(use_src)
        print src.get()

using source choice 0
gimme a number - 1
1.0
using source choice 1
0.702223268052
using source choice 2
Traceback (most recent call last):
  File "./stack26.py", line 28, in <module>
    print src.get()
AttributeError: 'BreakingSrc' object has no attribute 'get'

from abc import ABCMeta, abstractmethod
import random
import inspect

available_srcs = []

def register(newclass):
    if inspect.isabstract(newclass):
        print ("newclass %s is abstract, and has abstract"
                " methods: %s. Refusing to register"
                % (newclass, newclass.__abstractmethods__))
        return
    if newclass not in available_srcs:
        available_srcs.append(newclass)
        print "Registered %s as available source" % newclass

class MyMetaClass(ABCMeta):
    def __new__(cls, clsname, bases, attrs):
        newclass = super(MyMetaClass, cls).__new__(cls, clsname, bases, attrs)
        register(newclass)  # here is your register function
        return newclass

class BaseSource(object):
    __metaclass__ = MyMetaClass

    @abstractmethod
    def get(self):
        pass

class ManualSrc(BaseSource):
    def get(self):
        return float(raw_input('gimme a number - '))

class RandomSrc(BaseSource):
    def get(self):
        return random.random()

if __name__ == '__main__':
    for use_src in range(4):
        print 'using source choice {}'.format(use_src)
        src = available_srcs[use_src]()
        print src.get()

>>> import logging
>>> log = logging.getLogger('hello')
>>> vars(log)
{'name': 'hello', 'parent': <logging.RootLogger object at 0x17ce850>, 'handlers': [], 'level': 0, 'disabled': 0, 'manager': <logging.Manager object at 0x17ce910>, 'propagate': 1, 'filters': []}
>>> type(log)
<class 'logging.Logger'>

a = A()
 a.x = 5
 print "a.x is %s" % a.x

class B(object):
    def __init__(self):
        self.x = 10
    @property
    def x(self):
        return "I ain't returning x but something weird, and x is %s... FYI"
                % self._x
    @x.setter
    def x(self, x):
        self._x = int(self._x if hasattr(self, '_x') else 0 + 2 * x)

def A():
    return B()

>>> class C(object):
...     pass
...
>>> vars(C)
dict_proxy({'__dict__': <attribute '__dict__' of 'C' objects>, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'C' objects>, '__doc__': None})
>>> type(C)
<type 'type'>
>>> def get_me_a_c_class():
...     return C
...
>>> my_class = get_me_a_c_class()
>>> my_instance = my_class()
>>> type(my_instance)
<class '__main__.C'>
>>> type(get_me_a_c_class)
<type 'function'>
>>> vars(get_me_a_c_class)
{}
>>> get_me_a_c_class.random_attribute = 5
>>> print "Did I just put an attribute to a FUNCTION??: %s" % get_me_a_c_class.random_attribute
Did I just put an attribute to a FUNCTION??: 5