Example on how to merge class attribute through class hierarchy

inheritance.py

class DefaultsMetaBase(type):
  """Automatically merges headers from all parent classes."""
  def __call__(self, *args, **kwargs):
    obj = type.__call__(self)
    print("Metaclass %s called" % obj.__class__)
    for klass in obj.__class__.__mro__:
      if klass == obj.__class__ or klass == Base or not issubclass(klass, Base):
        print("discarding %s" % klass)
        continue
      print("keeping %s" % klass)
      if hasattr(klass, 'headers'):
        d = klass.headers.copy()
        d.update(obj.headers)
        obj.headers = d
    return obj


class Base(object):
    __metaclass__= DefaultsMetaBase


class A(Base):
  headers = {'a': 'default a', 'd': 'test d'}


class B(A):
  headers = {'b': 'default b'}


class C(A):
  headers = {'a': 'a overridden in C'}


class D(C):
  headers = {'d': 'd overridden in D'}


def test():
  a = A()
  b = B()
  c = C()
  d = D()
  print a.headers
  print b.headers
  print c.headers
  print d.headers
  assert (a.headers == {'a': 'default a', 'd': 'test d'})
  assert (b.headers == {'a': 'default a', 'b': 'default b', 'd': 'test d'})
  assert (c.headers == {'a': 'a overridden in C', 'd': 'test d'})
  assert (d.headers == {'a': 'a overridden in C', 'd': 'd overridden in D'})
  print("pass")

test()