An example on how to use metaclasses, difference between python 2 and python 3 and using six to write code supported on both.

metaclass-example1-py2.py

METACLASS_TXT = """
Hello, I am {} metaclass!
I am now creating '{}' class, child of '{}',
having the following class attributes:
{}
"""

class OCPRobotMeta(type):
    def __new__(cls, name, bases, attrs):
        """new is called for the creation of a new class"""

        print(METACLASS_TXT.format(cls, name, bases, attrs))

        return super(OCPRobotMeta, cls).__new__(cls, name, bases, attrs)


class Robocop(object):
    __metaclass__ = OCPRobotMeta

    prime_directives = ["Serve the public trust",
                        "Protect the innocent",
                        "Uphold the law"]


class Robocop2014(Robocop):
    pass


class ED209(object):
    __metaclass__ = OCPRobotMeta


class T800(object):
    pass


if __name__ == "__main__":
    robots = [Robocop(), Robocop2014(), ED209(), T800()]

    for r in robots:
        if isinstance(r.__class__, OCPRobotMeta):
            print "{} is a fine product of OCP Corp.".format(r.__class__)
        else:
            print "{} is not an OCP Corp. products".format(r.__class__)

"""
Output

Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'Robocop' class, child of '(<type 'object'>,)',
having the following class attributes:
{'__module__': '__main__', '__metaclass__': <class '__main__.OCPRobotMeta'>, 
    'prime_directives': ['Serve the public thrust', 'Protect the innocent', 'Uphold the law']}


Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'Robocop2014' class, child of '(<class '__main__.Robocop'>,)',
having the following class attributes:
{'__module__': '__main__'}


Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'ED209' class, child of '(<type 'object'>,)',
having the following class attributes:
{'__module__': '__main__', '__metaclass__': <class '__main__.OCPRobotMeta'>}

<class '__main__.Robocop'> is a fine product of OCP Corp.
<class '__main__.Robocop2014'> is a fine product of OCP Corp.
<class '__main__.ED209'> is a fine product of OCP Corp.
<class '__main__.T800'> is not an OCP Corp. products
"""

metaclass-example1-py3.py

METACLASS_TXT = """
Hello, I am {} metaclass!
I am now creating '{}' class, child of '{}',
having the following class attributes:
{}
"""

class OCPRobotMeta(type):
    def __new__(cls, name, bases, attrs):
        """new is called for the creation of a new class"""

        print(METACLASS_TXT.format(cls, name, bases, attrs))

        return super(OCPRobotMeta, cls).__new__(cls, name, bases, attrs)


class Robocop(metaclass=OCPRobotMeta):
    prime_directives = ["Serve the public trust",
                        "Protect the innocent",
                        "Uphold the law"]


class Robocop2014(Robocop):
    pass


class ED209(metaclass=OCPRobotMeta):
    pass


class T800:
    pass


if __name__ == "__main__":
    robots = [Robocop(), Robocop2014(), ED209(), T800()]

    for r in robots:
        if isinstance(r.__class__, OCPRobotMeta):
            print("{} is a fine product of OCP Corp.".format(r.__class__))
        else:
            print("{} is not an OCP Corp. products".format(r.__class__))

"""
Output


Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'Robocop' class, child of '()',
having the following class attributes:
{'prime_directives': ['Serve the public thrust', 'Protect the innocent', 
    'Uphold the law'], '__module__': '__main__', '__qualname__': 'Robocop'}


Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'Robocop2014' class, child of '(<class '__main__.Robocop'>,)',
having the following class attributes:
{'__module__': '__main__', '__qualname__': 'Robocop2014'}


Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'ED209' class, child of '()',
having the following class attributes:
{'__module__': '__main__', '__qualname__': 'ED209'}

<class '__main__.Robocop'> is a fine product of OCP Corp.
<class '__main__.Robocop2014'> is a fine product of OCP Corp.
<class '__main__.ED209'> is a fine product of OCP Corp.
<class '__main__.T800'> is not an OCP Corp. products
"""

metaclass-example1-six.py

import six

METACLASS_TXT = """
Hello, I am {} metaclass!
I am now creating '{}' class, child of '{}',
having the following class attributes:
{}
"""

class OCPRobotMeta(type):
    def __new__(cls, name, bases, attrs):
        """new is called for the creation of a new class"""

        super_new = super(OCPRobotMeta, cls).__new__

        # six.with_metaclass() inserts an extra class called 'NewBase' in the
        # inheritance tree: Model -> NewBase -> object. But the initialization
        # should be executed only once for a given model class.

        # attrs will never be empty for classes declared in the standard way
        # (ie. with the `class` keyword). This is quite robust.
        if name == 'NewBase' and attrs == {}:
            return super_new(cls, name, bases, attrs)

        print(METACLASS_TXT.format(cls, name, bases, attrs))

        return super_new(cls, name, bases, attrs)


class Robocop(six.with_metaclass(OCPRobotMeta)):
    prime_directives = ["Serve the public trust",
                        "Protect the innocent",
                        "Uphold the law"]


class Robocop2014(Robocop):
    pass


class ED209(six.with_metaclass(OCPRobotMeta)):
    pass


class T800:
    pass


if __name__ == "__main__":
    robots = [Robocop(), Robocop2014(), ED209(), T800()]

    for r in robots:
        if isinstance(r.__class__, OCPRobotMeta):
            print("{} is a fine product of OCP Corp.".format(r.__class__))
        else:
            print("{} is not an OCP Corp. products".format(r.__class__))

"""
Output (works both on python2 and python3)


Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'Robocop' class, child of '()',
having the following class attributes:
{'prime_directives': ['Serve the public thrust', 'Protect the innocent', 
    'Uphold the law'], '__module__': '__main__', '__qualname__': 'Robocop'}


Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'Robocop2014' class, child of '(<class '__main__.Robocop'>,)',
having the following class attributes:
{'__module__': '__main__', '__qualname__': 'Robocop2014'}


Hello, I am <class '__main__.OCPRobotMeta'> metaclass!
I am now creating 'ED209' class, child of '()',
having the following class attributes:
{'__module__': '__main__', '__qualname__': 'ED209'}

<class '__main__.Robocop'> is a fine product of OCP Corp.
<class '__main__.Robocop2014'> is a fine product of OCP Corp.
<class '__main__.ED209'> is a fine product of OCP Corp.
<class '__main__.T800'> is not an OCP Corp. products
"""