python - How do I create a simple metaclass? -

i've been doing python time now, , i've understood meaning of metaclasses, i've never needed one. think best solution problem metaclass (correct me if there's better way).

what i'm trying create system automatically adds class variable n , list instances each class of mine. here's simplified example of 1 class:

class foo:     n = 0     instances = []      def __init__(self):         self.index = foo.n         foo.n += 1         foo.instances.append(self) 

this structure should implemented 7 or 8 classes of mine, , thinking metaclass might me here. know can use foo.__metaclass__ = mymetaclass attribute use metaclass, how create metaclass?

actually, using base class work out better here:

class instanceslist(object):      def __new__(cls, *args, **kw):         if not hasattr(cls, 'instances'):             cls.instances = []         return super(instanceslist, cls).__new__(cls, *args, **kw)      def __init__(self):         self.index = len(type(self).instances)         type(self).instances.append(self)  class foo(instanceslist):     def __init__(self, arg1, arg2):         super(foo, self).__init__()         # foo-specific initialization