def import_from_json(self, data, *, override=False):
        """
        Import a JSON dictionary which must have the same format as exported by
        :meth:`export`.

        If *override* is true, the existing data in the pin store will be
        overriden with the data from `data`. Otherwise, the `data` will be
        merged into the store.
        """

        if override:
            self._storage = {
                hostname: set(self._decode_key(key) for key in pins)
                for hostname, pins in data.items()
            }
            return

        for hostname, pins in data.items():
            existing_pins = self._storage.setdefault(hostname, set())
            existing_pins.update(self._decode_key(key) for key in pins)