'Constructor.
Args:
prefix: A shared prefix for the configuration names being registered.
It *must* end in \'_\'.  (This is enforced by LibConfigRegistry.)
registry: A LibConfigRegistry instance.'

def __init__(self, prefix, registry):

    assert prefix.endswith('_')
    self._prefix = prefix
    self._defaults = {}
    self._overrides = {}
    self._registry = registry
    self._lock = threading.RLock()
