...also, since these variables only have relevance to the ppfield, why not define them within there? (again just a thought, might make it neater since the structure then describes where these things tie together).  This way, also we can refer to these objects from the subclasses of PPField:

```Python
class PPField(object):
    _NOCACHE_ELEMENT_NAMES = blabla
    _CACHABLE_ELEMENT_NAMES = blabla
    _V2_V3_HEADER_VARIANT_NAMES = blabla

    def __init__(self):
        .................................
```