'Returns a deterministic hash value of the WRITE_VERSION of all
registered transformers.'

@classmethod
@memoized
def get_write_version_hash(cls):

    hash_obj = sha1()
    sorted_transformers = sorted(cls.get_registered_transformers(), key=(lambda t: t.name()))
    for transformer in sorted_transformers:
        hash_obj.update(transformer.name().encode('utf-8'))
        hash_obj.update(str(transformer.WRITE_VERSION))
    return b64encode(hash_obj.digest())
