Question

from time import ctime
import inspect

class  MetaC(type):
  def  __init__(cls,name,bases,attrd):
    print   isinstance(cls,MetaC) 
    print   inspect.isclass(cls)
    super(MetaC,cls).__init__(name,bases,attrd)
    print "careated %s" %ctime()

class  Foo(object):
  __metaclass__=MetaC
  def  __init__(self):
    print "i am here "

I get this output:

True  
True  
careated Fri Feb  8 12:33:32 2013  

The argument(cls) in MetaC class __init__ method,

Is cls a class or an instance?

Answer 1

cls is both a class and an instance. A class is merely an instance of a meta class.

>>> class MetaC(type):
...   def  __init__(cls, name, bases, attrs):
...     print 'cls: ', cls
...     print 'cls is instance of MetaC: ', isinstance(cls, MetaC)
... 
>>> class C(object):
...   __metaclass__ = MetaC
... 
cls: <class '__main__.C'>
cls is instance of MetaC: True