def batch(self):
"""Factory to create a batch for this session.
:rtype: :class:`~google.cloud.spanner.batch.Batch`
:returns: a batch bound to this session
:raises ValueError: if the session has not yet been created.
"""
if self._session_id is None:
raise ValueError("Session has not been created.")
return Batch(self)
def __enter__(self):
"""Begin ``with`` block."""
session = self._session = self._database._pool.get()
batch = self._batch = Batch(session)
return batch