docs: def get(cls, pid_type, pid_value, pid_provider=None):
          """Get persistent identifier.
  
          :param pid_type: Persistent identifier type.
          :param pid_value: Persistent identifier value.
          :param pid_provider: Persistent identifier provider. (default: None).
          :raises: :exc:`invenio_pidstore.errors.PIDDoesNotExistError` if no
              PID is found.
          :returns: A :class:`invenio_pidstore.models.PersistentIdentifier`
              instance.
          """
          try:
              args = dict(pid_type=pid_type, pid_value=six.text_type(pid_value))
              if pid_provider:
                  args['pid_provider'] = pid_provider
              return cls.query.filter_by(**args).one()
          except NoResultFound:
              raise PIDDoesNotExistError(pid_type, pid_value)