Using dynamically generated subclasses.

ndmeta.py

class MetaMixinBase(object):
    # This will be overwritten by the type-creator
    _base = 'test'

    def propagate_add(self, other, kw):
        '''
        Empty for now but later this might contain the propagation of meta in addition.
        '''
        self._propagate_possible(other)
        return None

    def _propagate_possible(self, other):
        '''
        Verify if the two metas are allowed to be combined.
        '''
        if self._base != other._base:
            raise TypeError('Cannot propagate {0} with {1}'.format(self._base.__name__, other._base.__name__))

class MetaMixinCustom(MetaMixinBase):
    def propagate_add(self, other, kw):
        '''
        Create a copy of the meta and then replace the new ones.
        '''
        self._propagate_possible(other)
        ret = self._base(self)
        ret[kw] = self[kw] + other[kw]
        return ret

class NDDataOnlyMeta(object):
    '''
    Just very simulation of NDData with only data and meta just to verify it's working.
    '''
    def __init__(self, data, meta=None):
        self.data = data
        # Since we always have a dict I can skip the None-cases later on. :-)
        if meta is None:
            meta=OrderedDict()
        # Create a dynamically generated meta subclass
        meta = type('WithMixin',
                    (MetaMixinCustom, meta.__class__),
                    {'_base': meta.__class__})(meta)
        # Save it
        self.meta = meta

    def add(self, other, kw):
        return self.__class__(self.data+other.data, meta=self.meta.propagate_add(other.meta, kw))

# Showcase
from collections import OrderedDict

meta1 = {'a': 1, 'd': 2}
meta2 = OrderedDict({'c': 3, 'd': 4})
meta3 = {'c': 3, 'd': 4}
# Same type of meta: Should work
nd1 = NDDataOnlyMeta(5, meta=meta1)
nd2 = NDDataOnlyMeta(10, meta=meta3)
nd3 = nd1.add(nd2, 'd')
print(nd3.meta)
# Different types of meta: Shouldn't work
nd1 = NDDataOnlyMeta(5, meta=meta1)
nd2 = NDDataOnlyMeta(10, meta=meta2)
try:
    nd3 = nd1.add(nd2, 'd')
except TypeError:
    print("Should't work")