Camel is a library that lets you describe how to serialize your objects to YAML — and refuses to serialize them if you don't.

Quick example:

from camel import Camel, CamelRegistry
class DieRoll(tuple):
    def __new__(cls, a, b):
        return tuple.__new__(cls, [a, b])
    def __repr__(self):
        return "DieRoll(%s,%s)" % self
reg = CamelRegistry()
@reg.dumper(DieRoll, u'roll', version=None)
def _dump_dice(data):
    return u"{}d{}".format(*data)
@reg.loader(u'roll', version=None)
def _load_dice(data, version):
    a, _, b = data.partition(u'd')
    return DieRoll(int(a), int(b))
value = DieRoll(3, 6)
camel = Camel([reg])
print(camel.dump(value))
# !roll 3d6
# ...

Docs: http://camel.readthedocs.org/en/latest/

GitHub: https://github.com/eevee/camel