>>> class C(object):
...   def __init__(self, s):
...       print s
...
>>> myclass = C
>>> type(C)
<type 'type'>
>>> type(myclass)
<type 'type'>
>>> myclass(2)
2
<__main__.C object at 0x10e2bea50>
>>> map(myclass, [1,2,3])
1
2
3
[<__main__.C object at 0x10e2be9d0>, <__main__.C object at 0x10e2bead0>, <__main__.C object at 0x10e2beb10>]
>>> map(C, [1,2,3])
1
2
3
[<__main__.C object at 0x10e2be950>, <__main__.C object at 0x10e2beb50>, <__main__.C object at 0x10e2beb90>]
>>> C.test_attribute = True
>>> myclass.test_attribute
True

>>> def make_class(class_name):
...   class C(object):
...       def print_class_name(self):
...           print class_name
...   C.__name__ = class_name
...   return C
...
>>> C1, C2 = map(make_class, ["C1", "C2"])
>>> c1, c2 = C1(), C2()
>>> c1.print_class_name()
C1
>>> c2.print_class_name()
C2
>>> type(c1)
<class '__main__.C1'>
>>> type(c2)
<class '__main__.C2'>
>>> c1.print_class_name.__closure__
(<cell at 0x10ab6dbe8: str object at 0x10ab71530>,)

>>> MyClass = type("MyClass", (object,), {"my_attribute": 0})
>>> type(MyClass)
<type 'type'>
>>> o = MyClass()
>>> o.my_attribute
0

>>> def myclass_init(self, my_attr):
...   self.my_attribute = my_attr
...
>>> MyClass = type("MyClass", (object,), {"my_attribute": 0, "__init__": myclass_init})
>>> o = MyClass("Test")
>>> o.my_attribute
'Test'
>>> o.__init__
<bound method MyClass.myclass_init of <__main__.MyClass object at 0x10ab72150>>

>>> def mymetaclass(name, parents, attributes):
...   return "Hello"
...
>>> class C(object):
...   __metaclass__ = mymetaclass
...
>>> print C
Hello
>>> type(C)
<type 'str'>

def log_everything_metaclass(class_name, parents, attributes):
  print "Creating class", class_name
  myattributes = {}
  for name, attr in attributes.items():
    myattributes[name] = attr
    if hasattr(attr, '__call__'):
      myattributes[name] = logged("%b %d %Y - %H:%M:%S",
                    class_name + ".")(attr)
  return type(class_name, parents, myattributes)
 
class C(object):
  __metaclass__ = log_everything_metaclass
 
  def __init__(self, x):
    self.x = x
 
  def print_x(self):
    print self.x
 
# Usage:
print "Starting object creation"
c = C("Test")
c.print_x()
 
# Output:
Creating class C
Starting object creation
- Running 'C.__init__' on Aug 05 2013 - 13:50:58
- Finished 'C.__init__', execution time = 0.000s
- Running 'C.print_x' on Aug 05 2013 - 13:50:58
Test
- Finished 'C.print_x', execution time = 0.000s

def my_metaclass(class_name, parents, attributes):
  print "In metaclass, creating the class."
  return type(class_name, parents, attributes)
 
def my_class_decorator(class_):
  print "In decorator, chance to modify the class."
  return class_
 
@my_class_decorator
class C(object):
  __metaclass__ = my_metaclass
 
  def __init__(self):
    print "Creating object."
 
c = C()
 
# Output:
In metaclass, creating the class.
In decorator, chance to modify the class.
Creating object.

frametype_class_dict = {}
 
class ID3v2FrameClassFactory(object):
  def __new__(cls, class_name, parents, attributes):
    print "Creating class", class_name
    # Here we could add some helper methods or attributes to c
    c = type(class_name, parents, attributes)
    if attributes['frame_identifier']:
      frametype_class_dict[attributes['frame_identifier']] = c
    return c
 
  @staticmethod
  def get_class_from_frame_identifier(frame_identifier):
    return frametype_class_dict.get(frame_identifier)
 
class ID3v2Frame(object):
  frame_identifier = None
  __metaclass__ = ID3v2FrameClassFactory
  pass
 
class ID3v2TitleFrame(ID3v2Frame):
  __metaclass__ = ID3v2FrameClassFactory
  frame_identifier = "TIT2"
 
class ID3v2CommentFrame(ID3v2Frame):
  __metaclass__ = ID3v2FrameClassFactory
  frame_identifier = "COMM"
 
title_class = ID3v2FrameClassFactory.get_class_from_frame_identifier('TIT2')
comment_class = ID3v2FrameClassFactory.get_class_from_frame_identifier('COMM')
print title_class
print comment_class
 
# Output:
Creating class ID3v2Frame
Creating class ID3v2TitleFrame
Creating class ID3v2CommentFrame
<class '__main__.ID3v2TitleFrame'>
<class '__main__.ID3v2CommentFrame'>

frametype_class_dict = {}
 
class ID3v2FrameClass(object):
  def __init__(self, frame_id):
    self.frame_id = frame_id
 
  def __call__(self, cls):
    print "Decorating class", cls.__name__
    # Here we could add some helper methods or attributes to c
    if self.frame_id:
      frametype_class_dict[self.frame_id] = cls
    return cls
 
  @staticmethod
  def get_class_from_frame_identifier(frame_identifier):
    return frametype_class_dict.get(frame_identifier)
 
@ID3v2FrameClass(None)
class ID3v2Frame(object):
  pass
 
@ID3v2FrameClass("TIT2")
class ID3v2TitleFrame(ID3v2Frame):
  pass
 
@ID3v2FrameClass("COMM")
class ID3v2CommentFrame(ID3v2Frame):
  pass
 
title_class = ID3v2FrameClass.get_class_from_frame_identifier('TIT2')
comment_class = ID3v2FrameClass.get_class_from_frame_identifier('COMM')
print title_class
print comment_class
 
Decorating class ID3v2Frame
Decorating class ID3v2TitleFrame
Decorating class ID3v2CommentFrame
<class '__main__.ID3v2TitleFrame'>
<class '__main__.ID3v2CommentFrame'>

>>> def mydecorator(cls):
...   cls.__doc__ = "Test!"
...   return cls
...
>>> @mydecorator
... class C(object):
...   """Docstring to be replaced with Test!"""
...   pass
...
Traceback (most recent call last):
 File "<stdin>", line 2, in <module>
 File "<stdin>", line 2, in mydecorator
AttributeError: attribute '__doc__' of 'type' objects is not writable
>>> def mymetaclass(cls, parents, attrs):
...   attrs['__doc__'] = 'Test!'
...   return type(cls, parents, attrs)
...
>>> class D(object):
...   """Docstring to be replaced with Test!"""
...   __metaclass__ = mymetaclass
...
>>> D.__doc__
'Test!'

>>> class meta(type):
...   def __new__(cls, class_name, parents, attributes):
...       print "meta.__new__"
...       return super(meta, cls).__new__(cls, class_name, parents, attributes)
...   def __call__(self, *args, **kwargs):
...       print "meta.__call__"
...       return super(meta, self).__call__(*args, **kwargs)
...
>>> class C(object):
...   __metaclass__ = meta
...
meta.__new__
>>> c = C()
meta.__call__
>>> type(C)
<class '__main__.meta'>

class my_metaclass(type):
  def __new__(cls, class_name, parents, attributes):
    print "- my_metaclass.__new__ - Creating class instance of type", cls
    return super(my_metaclass, cls).__new__(cls,
                        class_name,
                        parents,
                        attributes)
 
  def __init__(self, class_name, parents, attributes):
    print "- my_metaclass.__init__ - Initializing the class instance", self
    super(my_metaclass, self).__init__(self)
 
  def __call__(self, *args, **kwargs):
    print "- my_metaclass.__call__ - Creating object of type ", self
    return super(my_metaclass, self).__call__(*args, **kwargs)
 
def my_class_decorator(cls):
  print "- my_class_decorator - Chance to modify the class", cls
  return cls
 
@my_class_decorator
class C(object):
  __metaclass__ = my_metaclass
 
  def __new__(cls):
    print "- C.__new__ - Creating object."
    return super(C, cls).__new__(cls)
 
  def __init__(self):
    print "- C.__init__ - Initializing object."
 
c = C()
print "Object c =", c

- my_metaclass.__new__ - Creating class instance of type <class '__main__.my_metaclass'>
- my_metaclass.__init__ - Initializing the class instance <class '__main__.C'>
- my_class_decorator - Chance to modify the class <class '__main__.C'>
- my_metaclass.__call__ - Creating object of type <class '__main__.C'>
- C.__new__ - Creating object.
- C.__init__ - Initializing object.
Object c = <__main__.C object at 0x1043feb90> <class '__main__.C'>