class PolicyClass(type): # All metaclasses inherit type.
__dict = {} # Used to store created classes.
def __new__(cls, name, bases, d): # class, name, bases, dict
def __make(*T): # Wrapper function for old class.
try: # T* = Bases of new class.
this = cls.__dict[(name, T)] # Make sure the class doesn't
except KeyError: # already exist.
this = type(name, # Construct new class using type.
tuple([C for C in # type() only accepts tuples.
(bases + T) # Add policies to original bases.
if C is not type]), d) # Remove <type> from bases.
cls.__dict[(name, T)] = this # Store new class for later use.
return this # Return resulting class.
return __make # Replace original class.
# examples --
class Class(type):
__metaclass__ = PolicyClass
C = 'X'
def __init__(self): # super(Class, self) won't work
super( # because Class has already
self.__class__, self # been replaced with
).__init__() # PolicyClass.__new__.__make.
self.c = 'x'
class PolicyA(object):
A = 'Y'
def __init__(self):
super(PolicyA, self).__init__()
self.a = 'y'
class PolicyB(object):
B = 'Z'
def __init__(self):
super(PolicyB, self).__init__()
self.b = 'z'
ClassA = Class(PolicyA, PolicyB)
ClassB = Class(PolicyB, PolicyA)
assert ClassA is Class(PolicyA, PolicyB) # PolicyClass.__dict allows this.
assert ClassA is not ClassB # These are NOT the same.
Instance = ClassA()
assert Instance.A.lower() == Instance.a # Ensure original class works.
assert Instance.B.lower() == Instance.b # Ensure inheritance works.
assert Instance.C.lower() == Instance.c # Ensure MRO works.
